Apparatus and method for creating, maintaining, and controlling a virtual electrode used for the ablation of tissue

ABSTRACT

The present invention provides an apparatus and a method for producing a virtual electrode within or upon a tissue to be treated with radio frequency alternating electric current, such tissues including but not limited to liver, lung, cardiac, prostate, breast, and vascular tissues and neoplasms. An apparatus in accord with the present invention includes a supply of a conductive or electrolytic fluid to be provided to the patient, an alternating current generator, and a processor for creating, maintaining, and controlling the ablation process by the interstitial or surficial delivery of the fluid to a tissue and the delivery of electric power to the tissue via the virtual electrode. A method in accord with the present invention includes delivering a conductive fluid to a predetermined tissue ablation site for a predetermined time period, applying a predetermined power level of radio frequency current to the tissue, monitoring at least one of several parameters, and adjusting either the applied power and/or the fluid flow in response to the measured parameters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/699,548, filed Oct. 31, 2003, now U.S. Pat. No. 7,169,144, which is acontinuation of U.S. patent application Ser. No. 09/903,296, filed onJul. 11, 2001, now U.S. Pat. No. 6,736,810, which is a continuation ofU.S. application Ser. No. 09/347,635, filed on Jul. 6, 1999, now U.S.Pat. No. 6,409,722 entitled “Apparatus and Method For Creating,Maintaining, and Controlling a Virtual Electrode Used For the Ablationof Tissue”, which claims the benefit of priority under 35 U.S.C. §119(e)to U.S. Provisional Application Ser. No. 60/091,959, filed on Jul. 7,1998.

This application is also a continuation-in-part of U.S. patentapplication Ser. No. 10/411,921, filed Apr. 11, 2003, now U.S. Pat. No.6,764,487, which is a continuation of U.S. patent application Ser. No.09/955,496, filed Sep. 18, 2001, now U.S. Pat. No. 6,585,732 entitled“Fluid-Assisted Electrosurgical Device”, which is a continuation of U.S.patent application Ser. No. 09/580,228, filed May 26, 2000 , now U.S.Pat. No. 6,358,248, which is a continuation of U.S. patent applicationSer. No. 09/236,034, filed Jan. 22, 1999, now abandoned, which is acontinuation of U.S. patent application Ser. No. 08/556,784, filed Nov.2, 1995, now U.S. Pat. No. 5,897,553, which is a continuation-in-part ofU.S. patent application Ser. No. 08/393,082, filed Feb. 22, 1995, nowU.S. Pat. No. 6,063,081 entitled “Fluid-Assisted Electrocautery Device”.This application claims the benefit of priority of the aboveapplications and incorporates the entire disclosure of each applicationby reference to the extent they are consistent.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus and method ofthe creation of a virtual electrode. More particularly, the presentinvention relates to an apparatus and method of the creation of avirtual electrode, which is useful for the ablation of soft tissue andneoplasms.

BACKGROUND OF THE PRESENT INVENTION

The utilization of an electric current to produce an ameliorative effecton a bodily tissue has a long history, reportedly extending back to theancient Greeks. The effects on bodily tissue from an applied electriccurrent, and thus the dividing line between harmful and curativeeffects, will vary depending upon the voltage levels, current levels,the length of time the current is applied, and the tissue involved. Onesuch effect resulting from the passage of an electric current throughtissue is heat generation.

Body tissue, like all non-superconducting materials, conducts currentwith some degree of resistance. This resistance creates localizedheating of the tissue through which the current is being conducted. Theamount of heat generated will vary with the power P deposited in thetissue, which is a function of the product of the square of the currentI and the resistance R of the tissue to the passage of the currentthrough it (P=I²R.).

As current is applied to tissue, then, heat is generated due to theinherent resistance of the tissue. Deleterious effects in the cellsmaking up the tissue begin to occur at about 42° Celsius. As thetemperature of the tissue increases because of the heat generated by thetissue's resistance, the tissue will undergo profound changes andeventually, as the temperature becomes high enough, that is, generallygreater than 45° C., the cells will die. The zone of cell death is knownas a lesion and the procedure followed to create the lesion is commonlycalled an ablation. As the temperature increases beyond cell deathtemperature, complete disintegration of the cell walls and cells causedby boiling off of the tissue's water can occur. Cell death temperaturescan vary somewhat with the type of tissue to which the power is beingapplied, but generally will begin to occur within the range of 45° to60° C., though actual cell death of certain tissue cells may occur at ahigher temperature.

In recent times, electric current has found advantageous use in surgery,with the development of a variety of surgical instruments for cuttingtissue or for coagulating blood. Still more recently, the use ofalternating electric current to ablate, that is, kill, various tissueshas been explored. Typically, current having a frequency from about 3kilohertz to about 300 gigahertz, which is generally known asradiofrequency or radiofrequency (RF) current, is used for thisprocedure. Destruction, that is, killing, of tissue using an RF currentis commonly known as radiofrequency ablation. Often radiofrequencyablation is performed as a minimally invasive procedure and is thusknown as radiofrequency catheter ablation because the procedure isperformed through and with the use of a catheter. By way of example,radiofrequency catheter ablation has been used to ablate cardiac tissueresponsible for irregular heartbeat arrhythmias.

The prior art applications of current to tissue have typically involvedapplying the current using a “dry” electrode. That is, a metal electrodeis applied to the tissue desired to be affected and a generated electriccurrent is passed through the electrode to the tissue. A commonly knownexample of an instrument having such an operating characteristic is anelectrosurgical instrument known as a “bovie” knife. This instrumentincludes a cutting/coagulating blade electrically attached to a currentgenerator. The blade is applied to the tissue of a patient and thecurrent passes through the blade into the tissue and through thepatient's body to a metal base electrode or ground plate usually placedunderneath and in electrical contact with the patient. The baseelectrode is in turn electrically connected to the current generator soas to provide a complete circuit.

As the current from the bovie knife passes from the blade into thetissue, the resistance provided by the tissue creates heat. In thecutting mode, a sufficient application of power through the bovie to thetissue causes the fluid within the cell to turn to steam, creating asufficient overpressure so as to burst the cell walls. The cells thendry up, desiccate, and carbonize, resulting in localized shrinking andan opening in the tissue. Alternatively, the bovie knife can be appliedto bleeding vessels to heat and coagulate the blood flowing therefromand thus stop the bleeding.

As previously noted, another use for electrical instruments in thetreatment of the body is in the ablation of tissue. To expand further onthe brief description given earlier of the ablation of cardiac tissue,it has long been known that a certain kind of heart tissue known assino-atrial and atrio-ventricular nodes spontaneously generate anelectrical signal that is propagated throughout the heart alongconductive pathways to cause it to beat. Occasionally, certain hearttissue will “misfire,” causing the heart to beat irregularly. If theerrant electrical pathways can be determined, the tissue pathways can beablated and the irregular heartbeat remedied. In such a procedure, anelectrode is placed via a catheter into contact with the tissue and thencurrent is applied to the tissue via the electrode from a generator ofRF current. The applied current will cause the tissue in contact withthe electrode to heat. Power will continue to be applied until thetissue reaches a temperature where the heart tissue dies, therebydestroying the errant electrical pathway and the cause of the irregularheartbeat.

Another procedure using RF ablation is transurethral needle ablation, orTUNA, which is used to create a lesion in the prostate gland for thetreatment of benign prostatic hypertrophy (BPH) or the enlargement ofthe prostate gland. In a TUNA procedure, a needle having an exposedconductive tip is inserted into the prostate gland and current isapplied to the prostate gland via the needle. As noted previously, thetissue of the prostate gland heats locally surrounding the needle tip asthe current passes from the needle to the base electrode. A lesion iscreated as the tissue heats and the destroyed cells may be reabsorbed bythe body, infiltrated with scar tissue, or just become non-functional.

While there are advantages and uses for such “dry” electrodeinstruments, there are also several notable disadvantages. One of thesedisadvantages is that during a procedure coagulum—dried blood cells andtissue cells—will form on the electrode engaging the tissue. Coagulumacts as an insulator and effectively functions to prevent currenttransfer from the blade to the tissue. This coagulum “insulation” can beovercome with more voltage so as to keep the current flowing, but onlyat the risk of arcing and injuring the patient. Thus, during surgerywhen the tissue is cut with an electrosurgical scalpel, a build-up ofcoagulated blood and desiccated tissue will occur on the blade,requiring the blade to be cleaned before further use. Typically,cleaning an electrode/scalpel used in this manner will involve simplyscraping the dried tissue from the electrode/scalpel by rubbing thescalpel across an abrasive pad to remove the coagulum. This is a tediousprocedure for the surgeon and the operating staff since it requires the“real” work of the surgery to be discontinued while the cleaningoperation occurs. This procedure can be avoided with the use ofspecially coated blades that resist the build up of coagulum. Suchspecialty blades are costly, however.

A second disadvantage of the dry electrode approach is that theelectrical heating of the tissue creates smoke that is now known toinclude cancer-causing agents. Thus, preferred uses of such equipmentwill include appropriate ventilation systems, which can themselvesbecome quite elaborate and quite expensive.

A further, and perhaps the most significant, disadvantage of dryelectrode electrosurgical tools is revealed during cardiac ablationprocedures. During such a procedure, an electrode that is otherwiseinsulated but having an exposed, current carrying tip is inserted intothe heart chamber and brought into contact with the inner or endocardialside of the heart wall where the ablation is to occur. The current isinitiated and passes from the current generator to the needle tipelectrode and from there into the tissue so that a lesion is created.Typically, however, the lesion created by a single insertion isinsufficient to cure the irregular heartbeat because the lesion createdis of an insufficient size to destroy the errant electrical pathway.Thus, multiple needle insertions and multiple current applications arealmost always required to ablate the errant cardiac pathway, prolongingthe surgery and thus increasing the potential risk to the patient.

This foregoing problem is also present in TUNA procedures, whichsimilarly requires multiple insertions of the needle electrode into theprostate gland. Failing to do so will result in the failure to create alesion of sufficient size such that the procedure produces a beneficialresult. As with radiofrequency catheter ablation of cardiac tissue,then, the ability to create a lesion of the necessary size to alleviateBPH symptoms is limited and thus requires multiple insertions of theelectrode into the prostate.

A typical lesion created with a dry electrode using RF current and asingle insertion will normally not exceed one centimeter in diameter.This small size—often too small to be of much or any therapeuticbenefit—stems from the fact that the tissue surrounding the needleelectrode tends to desiccate as the temperature of the tissue increases,leading to the creation of a high resistance to the further passage ofcurrent from the needle electrode into the tissue, all as previouslynoted with regard to the formation of coagulum on an electrosurgicalscalpel. This high resistance—more properly termed impedance sincetypically an alternating current is being used—between the needleelectrode and the base electrode is commonly measured by the RF currentgenerator. When the measured impedance reaches a pre-determined level,the generator will discontinue current generation. Discontinuance of theablation procedure under these circumstances is necessary to avoidinjury to the patient.

Thus, a typical procedure with a dry electrode may involve placing theneedle electrode at a first desired location; energizing the electrodeto ablate the tissue; continue applying current until the generatormeasures a high impedance and shuts down; moving the needle to a newlocation closely adjacent to the first location; and applying currentagain to the tissue through the needle electrode. This cycle ofelectrode placement, electrode energization, generator shut down,electrode re-emplacement, and electrode re-energization, will becontinued until a lesion of the desired size has been created. As noted,this increases the length of the procedure for the patient.Additionally, multiple insertions increases the risk of at least one ofthe placements being in the wrong location and, consequently, the riskthat healthy tissue may be undesirably affected while diseased treatmentmay be left untreated. The traditional RF ablation procedure of using adry ablation therefore includes several patient risk factors that bothpatient and physician would prefer to reduce or eliminate.

The therapeutic advantages of RF current could be increased if a largerlesion could be created safely with a single positioning of thecurrent-supplying electrode. A single positioning would allow theprocedure to be carried out more expeditiously and more efficiently,reducing the time involved in the procedure. Larger lesions can becreated in at least two ways. First, simply continuing to apply currentto the patient with sufficiently increasing voltage to overcome theimpedance rises will create a larger lesion, though almost always withundesirable results to the patient. Second, a larger lesion can becreated if the current density, that is, the applied electrical energy,could be spread more efficiently throughout a larger volume of tissue.Spreading the current density over a larger tissue volume wouldcorrespondingly cause a larger volume of tissue to heat in the firstinstance. That is, by spreading the applied power throughout a largertissue volume, the tissue would heat more uniformly over a largervolume, which would help to reduce the likelihood of generator shutdowndue to high impedance conditions. The applied power, then, will causethe larger volume of tissue to be ablated safely, efficiently, andquickly.

Research conducted under the auspices of the assignee of the presentinvention has focused on spreading the current density throughout alarger tissue volume through the creation, maintenance, and control of a“virtual electrode” within or adjacent to the tissue to be ablated. Avirtual electrode can be created by the introduction of a conductivefluid, such as isotonic or hypertonic saline, into or onto the tissue tobe ablated. The conductive fluid will facilitate the spread of thecurrent density substantially equally throughout the extent of the flowof the conductive fluid, thus creating an electrode—a virtualelectrode—substantially equal in extent to the size of the deliveredconductive fluid. RF current can then be passed through the virtualelectrode into the tissue.

A virtual electrode can be substantially larger in volume than theneedle tip electrode typically used in RF interstitial ablationprocedures and thus can create a larger lesion than can a dry, needletip electrode. That is, the virtual electrode spreads or conducts the RFcurrent density outward from the RF current source—such as a currentcarrying needle, forceps or other current delivery device—into or onto alarger volume of tissue than is possible with instruments that rely onthe use of a dry electrode. Stated otherwise, the creation of thevirtual electrode enables the current to flow with reduced resistance orimpedance throughout a larger volume of tissue, thus spreading theresistive heating created by the current flow through a larger volume oftissue and thereby creating a larger lesion than could otherwise becreated with a dry electrode.

While the efficacy of RF current ablation techniques using a virtualelectrode has been demonstrated in several studies, the currentlyavailable instruments useful in such procedures lags behind the researchinto and development of hoped-for useful treatment modalities for theablation of soft tissue and malignancies. Thus, to perform currentresearch procedures it is necessary to provide separately both a fluidpump and an RF current generator. The fluid pump requires an electricalconnection and fluid conduits running from the fluid supply to the fluiddelivery instrument. The generator also requires electrical connectionsand electric lines running from the generator to the surgicalinstrument, as well as a return electrical line from the ground platewhen a monopolar electrode is used. Use of these systems is thushampered by the many fluid and electrical lines surrounding theoperating theater, all of which can easily become entangled andcomplicate their use.

Further, currently available generators provide limited control over thepower application to the tissue. For example, such generators, which areoften designed for cardiac ablation procedures, provide for an immediateor nearly immediate cessation of power upon the occurrence of equipmentdefined high impedance conditions. These generators are thus unable tooperate continuously—and therefore unable to provide the mostexpeditious, efficient therapy—when operating in the presence offleeting high impedance conditions. Consequently, the presentlyavailable generators will automatically shut off when such a conditionoccurs even though continued application of RF power would otherwise besafe. The generator must then be restarted, which is not only anunnecessary annoyance but also prolongs the procedure since suchshutdowns can occur more than once during any one ablation procedure.

It would be desirable to have an apparatus and method capable ofcreating, maintaining, and controlling a virtual electrode whileproviding a controlled application of tissue ablating RF electriccurrent to a tissue of interest so as to produce a lesion of desiredsize and configuration. Preferably, such an apparatus will be capable ofadjusting the applied current and fluid flow in accord with the measuredimpedance of the tissue and/or the temperature of the tissue beingablated. It would also be desirable to have such an apparatus and methodcapable of continuing operation in the presence of transient highimpedance conditions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new and improvedapparatus and method that is not subject to the foregoing disadvantages.

It is another object of the present invention to provide an integratedconductive fluid supply and radio frequency current generator.

It is still another object of the present invention to provide a devicehaving an integrally controlled and operated conductive fluid supply andradio frequency current generator that controls the applied power andrate of conductive fluid infusion in response to the measured impedanceof the tissue being ablated.

It is yet another object of the present invention to provide anintegrally controlled and operated conductive fluid supply and radiofrequency current generator that controls the applied power and rate ofconductive fluid infusion in response to the measured impedancetransients.

It is still yet another object of the present invention to provide anintegrally controlled and operated conductive fluid supply and radiofrequency current generator that selectively controls the applied powerand rate of conductive fluid infusion in response to the measuredtemperature of the tissue being ablated.

It is a further object of the present invention to provide a method fortreating a patient with RF current via a virtual electrode to produce alesion of a predetermined size.

The foregoing objects of the present invention are achieved by anapparatus and a method for producing a virtual electrode within or upona tissue to be treated with radio frequency alternating electriccurrent, such tissues including but not limited to liver, lung, cardiac,prostate, breast, and vascular tissues and neoplasms. An apparatus inaccord with the present invention will include a supply of a conductiveor electrolytic fluid to be provided to the patient, an alternatingcurrent generator, and a processor for creating, maintaining, andcontrolling the ablation process by the interstitial or surficialdelivery of the fluid to a tissue and the delivery of electric power tothe tissue via the virtual electrode.

The control of the virtual electrode and the ablation procedure isaccomplished in response to measured temperatures at pre-determineddistances from a current delivery device and/or measured impedances overpre-determined time intervals. The ablation procedure is adjusted due totransient impedance rises related to the novel apparatus and itsoperation as well as due to those impedance and temperature changesrelated to the flow of current from the virtual electrode through thetissue being treated. Such an apparatus in accord with the presentinvention will also preferably include a display for visualizingpredetermined operational parameters and for visualizing the ongoingoperation of the virtual electrode and the apparatus during an ablationprocedure.

In addition, an apparatus in accord with the present invention will alsopreferably include an input apparatus such as a rotary encoder andmouse-type of device, a touch screen useful in the form of a menu-drivenicon system, and/or a keyboard. The input apparatus is provided to allowthe operator to input predetermined information, such as tissue type tobe ablated, desired lesion size, type of electrolytic fluid being used,the particular surgical instrument being used, and any other desiredparameter of interest.

An apparatus in accord with the present invention may also include oneor more additional fluid administration systems providing a controlleddelivery of at least a second fluid to the patient during an ablationprocedure. Such a fluid may be provided for maintaining the temperatureof the tissue surrounding the tissue to be ablated at a non-harmfultemperature, such as by directly cooling the tissue. Alternatively, aninsulating fluid such as dextrose may be infused into the tissuesurrounding the tissue ablation site, thus increasing the impedance ofthis surrounding tissue to prevent current passage therethrough andthereby diminishing the likelihood of unwanted heating of thissurrounding tissue.

A method in accord with the present invention will include the steps ofdelivering a conductive fluid to a predetermined tissue ablation site tobe ablated for a predetermined time period that may range from zeroseconds to about ten minutes, preferably within the range of zero tosixty seconds, thereby creating a virtual electrode, applying apredetermined power level of radio frequency current to the tissue viathe virtual electrode, monitoring at least one of several parameters(including but not limited to impedance, tissue temperature at one ormore pre-selected distances from the current delivery device, theapplied power, and fluid flow), and adjusting either the applied powerand/or the fluid flow in response to the measured parameters. The methodmay also include the placement of a current delivery device within thetissue ablation site, such as substantially at the center thereof,and/or the placement of at least one additional temperature sensor intissue whose ablation is not desired yet is adjacent to thepredetermined ablation site. By way of example only, where an increaseof impedance of predetermined size occurs within a predetermined timeperiod the applied power may be automatically reduced by a predeterminedamount for a predetermined period of time. By way of yet anotherexample, when a tissue target temperature is reached, the applied powermay be reduced to maintain the temperature at the target temperature andreduce the likelihood of increasing the tissue temperature of theablation site above the target temperature. By way of another example,the flow of conductive fluid to the tissue ablation site can beincreased or decreased in response to one or more of the measuredparameters (temperature, impedance, etc.).

The foregoing objects of the invention will become apparent to thoseskilled in the art when the following detailed description of theinvention is read in conjunction with the accompanying drawings andclaims. Throughout the drawings, like numerals refer to similar oridentical parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in an overview of an apparatus in accord with thepresent invention in an operational setting with a human patient;

FIG. 2 shows an example of a housing, display, and controls which may beused with the present invention;

FIG. 3 is a schematic representation of an apparatus in accord with thepresent invention;

FIG. 4 schematically illustrates display screens forming a user oroperator interface for use in an apparatus in accord with the presentinvention;

FIG. 5 depicts in general schematic outline the various parameters thatmay be measured and controlled by the present invention;

FIG. 6 shows schematically the interaction of the various control loopsthat may be implemented in accord with the present invention;

FIG. 7 illustrates the various inputs and parameters used in the controland operation of the RF control loops in accord with the presentinvention;

FIG. 8 depicts generally the Fluid Flow Control Loop and the inputs andoutputs used in the control of the flow of conductive fluid;

FIG. 9 schematically illustrates the software blocks contained withinthe microprocessor shown in FIG. 3;

FIG. 10 illustrates schematically the operation of an apparatus inaccord with the present invention;

FIG. 11 illustrates a pre-ablation operating routine of an apparatus inaccord with the present invention;

FIG. 12 illustrates an arc detection loop of an apparatus in accord withthe present invention;

FIG. 13 illustrates a method in accord with the present invention forcontrolling the application of radio frequency power to a patient basedupon detected electrical arcing;

FIG. 14 illustrates an impedance control loop of an apparatus in accordwith the present invention;

FIG. 15 illustrates a temperature control loop of an apparatus in accordwith the present invention;

FIG. 16 illustrates a method in accord with the present invention usingmeasured impedance to control the fluid flow rate and the applied power;

FIG. 17 illustrates a method in accord with the present invention usingmeasured temperatures to control the fluid flow rate and the appliedpower;

FIG. 18 illustrates the present invention in operation with respect to apatient.

FIG. 19 shows an enlarged view of a surgical instrument useful with thepresent invention;

FIG. 20 shows a virtual electrode created by the present invention;

FIG. 21 is a graph showing an example of a fluid flow control functionuseful in accord with the present invention;

FIG. 22 is a graph showing another example of a fluid flow controlfunction useful in accord with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-4, a Virtual Electrode Thermal Ablation Device(VETAD) 10 in accord with the present invention will be described. VETAD10 comprises a housing 12 for receiving a conductive fluid supply 14, agenerator 18 for generating radio frequency current, a microprocessor 20for, among other features, substantially simultaneously controlling theflow rate of fluid 14 supplied by pump 16 and the power supplied bygenerator 18, and a display/control panel 22.

Conductive fluid is transferred from supply 14 via a fluid line 24 to asurgical instrument 26 and then to a patient 28. A pump 16, which ispreferably a syringe-type pump of the kind presently manufactured byKloehn Corporation, may be used to assist in the transfer of conductivefluid from the fluid supply to the patient. A syringe-type pumpgenerally includes a piston 17 that will extend in the direction of thearrow 17 a to engage the syringe plunger 17 b of a syringe 17 c holdingthe conductive fluid to be used in the RF therapy procedure. Syringe 17c is held within a recess 17 d of housing 12, the recess 17 d beingconfigured to accept syringes having standard configurations. Duringoperation of VETAD 10 the piston 17 will be moved to engage the syringeplunger 17 b and push the plunger 17 b through the syringe barrel 17 eof syringe 17 c, thus forcing the conductive fluid out of the syringe 17c into the fluid line 24. It will be observed that the syringe 17 c isdisposed such that the extension of the piston 17 is generallyhorizontal, though other orientations would work equally well with thepresent invention.

More generally and preferably, pump 16 is of the type where the fluid inthe supply 14 does not come into contact with the mechanisms of the pump16. With this type of pump the sterility of the conductive fluid can bemaintained until delivered to the patient 28. Another such type of pump,then, that could also be used with the present invention is a flexibletube pump. The fluid line 24 will generally thus take the form of aflexible tube that extends either from the supply 14 to the instrument26 if pump 16 is a syringe-type pump, or from the supply 14 through thepump 16, and out to the surgical instrument 26 if pump 16 is a flexibletube type of pump. As part of a method of treating a patient accordingto the present invention, then, the fluid supply 14 may comprise acontainer, such as a syringe or flexible bag, holding a conductive orelectrolytic fluid, such as isotonic or hypertonic saline. Desirably,the fluid supply 14 and the fluid line 24 will be supplied as new,sterile, disposable equipment for each procedure, thereby substantiallyreducing the risk of contamination of the fluid or of infecting thepatient. With a syringe pump, the fluid supply will comprise a syringefilled with a conductive fluid. The syringe may be pre-filled or filledmanually by a clinician or the like prior to an ablation procedure.

Generator 18 will supply radio frequency alternating current to thesurgical instrument 26 via electrical line 30. The operation ofgenerator 18 is controlled by microprocessor 20, which communicates withgenerator 18 as will be described below. Microprocessor 20 alsocommunicates with pump 16 via a line 34. Microprocessor 20 is in turncontrolled by its preprogrammed software and by the inputs received fromthe display/control panel 22 over line 36.

Surgical instrument 26 may take various forms. Minimally, however, theinstrument 26 will be capable of delivering surficially orinterstitially an RF current ablating fluid to create the virtualelectrode and will include means for applying the current to the tissuevia the virtual electrode. For example, instrument 26 may comprise astraight metal needle having an interior fluid transmitting lumen. Sucha needle will be insulated except for a pre-determined length at thedistal or patient end thereof and will have a single aperture at thedistal end and/or one or more apertures disposed around the needle shaftfor the infusion of the conductive fluid 14 from the needle lumen intothe tissue to create the virtual electrode. In addition, surgicalinstrument 26 can comprise other forms of instruments useful in thedelivery of both radio frequency current and a conductive fluid to apatient. Such instruments would include needles of variousconfigurations, catheter or guide wire assemblies, blades, and forceps,as well as any other future developed instruments that can supply aconductive fluid to the tissue to be ablated along with RF currentapplied either to the tissue and/or the conductive fluid.

A VETAD 10 in accord with the present invention will preferably have thecapability of providing a power output of 0.1 watt to about 200 watts ata frequency of between about 350 kHz and 700 KHZ, preferably about 475KHZ, and of incrementing or decrementing the power output by about 1watt intervals. VETAD 10 will preferably be able to deliver the desiredpower level into resistive loads in a range of about ten (10) ohms toabout five hundred (500) ohms. In addition to creating a virtualelectrode and providing an ablative radio frequency current to a patientvia the virtual electrode, VETAD 10 will monitor a variety of variablesinvolved in an RF current ablation procedure, including but not limitedto (1) the temperature of the tissue being ablated or surrounding thetissue being ablated via one or more thermocouples; (2) the occurrenceof an electrical arc detected by a spike in the measured voltage; (3)the applied power; (4) the flow rate of the conductive fluid; and (5)the impedance between the virtual electrode and the ground or baseplate.

Pump 16 of VETAD 10 will deliver conductive fluid to the ablation siteat flow rates of about 0.1 to 10.0 cubic centimeters per minute inincrement sizes of 0.1 cubic centimeters per minute. Where a syringepump is used, VETAD 10 will be configured so that the amount ofconductive fluid remaining in the barrel of the syringe can bedetermined via the relative position of the piston 17 of the pump. Inone embodiment of the present invention, the VETAD 10 will provide anindication of low fluid volume remaining when piston travel has reachedninety percent (90%) of maximum travel and when the fluid has beenexhausted as indicated by the piston travel reaching the one hundredpercent (100%) maximum travel distance.

As illustrated in FIG. 2, the housing 12 will include separate ports forthe fluid line 24 and the line 30 (desirably forming part of line 91 aswill be explained below). If desired, however, a single port forprovision of both the conductive fluid and the radio frequency currentto the instrument 26 could be provided. Furthermore, it should be notedthat FIGS. 1 and 3 illustrate the use of a monopolar surgical instrument26 in association with VETAD 10. With such a use, a ground pad 38 willbe placed in electrical contact with the patient 26 and a line 40 willextend between the ground pad 38 and VETAD 10 so as to provide acomplete electrical circuit from VETAD 10 to the patient 28 and back toVETAD 10 for the current produced by VETAD 10. More generally, theground pad provides a return electrode completing the electricalcircuit. Bi-polar instruments are also known in the art and when such abi-polar instrument would be used with the present invention 10 thecomplete electrical circuit would be provided by a return electrodewithin the instrument 26. With such a bipolar instrument a return pathfor the RF current from the generator 18 would be provided by the returnelectrode within instrument 26 and line 30.

The connectors used to couple the instrument 26 to VETAD 10 can takeseveral different forms. For example, the pin connections betweeninstrument 26 and VETAD 10 could provide the VETAD 10 with anidentification. Varying surgical instruments could then use differentpins to indicate to VETAD 10 which instrument is being used. By way ofexample only, a nine-pin port could be used and the surgical instrumentscould use one or more of the pins to communicate with the VETAD 10 andprovide its identity and/or default operating parameters. VETAD 10 wouldthen establish the initial operating parameters for an ablationprocedure based upon the instrument's identity—for example, a straightneedle versus a helical needle. That is, with this form of instrument,the default and desired operating parameters would be stored in thememory associated with the microprocessor 20 for a particular disposablesurgical instrument 26, the microprocessor “recognizing” the disposableby the pin configuration thereof.

Alternatively, a standard connector could be used for all disposablesurgical instruments 26. Each instrument 26 can then include apre-programmed microchip or memory chip 42 that would communicate withthe microprocessor 20 over a line 44 and that would provide VETAD 10with identifying characteristics and other relevant information such asdefault operating parameters relating to power, temperature, and fluidflow upon receiving the appropriate inquiry signal from VETAD 10.Providing the instruments 26 with a memory chip 42 will also enable eachinstrument 26 to be given a unique identifier that the microprocessor 20can use for tracking the number of uses of the particular instrument 26.For example, the microprocessor can be programmed to limit the total ofnumber of times that RF current is sent through the instrument, thetotal amount of time that the instrument 26 is subjected to RF current,and the time frame within which an, instrument 26 is used. By way ofexample only and not to limit the present invention in any manner, thedefaults for a particular instrument 26 may be 10 distinct start-ups, atotal of 10 minutes of ablation time, and a four hour time period inwhich the instrument must be used. The instruments 26 may include on/offswitches 46 that would communicate with microprocessor 20 to allow thesurgeon to start/stop the procedure and/or control the RF power during aprocedure. Regardless of the form of the connection between theinstruments 26 and VETAD 10 then, instrument 26 and its connector toVETAD 10 will preferably be disposable, with the disposal time beingdetermined based upon the number of ablation procedures begun or theexpiration of a predetermined time period following first use of theinstrument. This will facilitate the use of sterile procedures andreduce the risk of secondary infections to the patient.

Referring now to FIGS. 2 and 3, VETAD 10 will include a power on/offswitch 48 for VETAD 10 and an on or start switch 50 connected tomicroprocessor 20 by a line 52 and an off or stop switch 54 connected tomicroprocessor 20 by a line 56 and to RF generator 18 by a line 60.Thus, switch 48 will turn the apparatus 10 on while switch 50 willinitiate the flow of RF current from the generator 18 to the patient 28.In addition, VETAD 10 may include a foot pedal 62 connected tomicroprocessor 20 by a line 64. Switch 54 will also function as anemergency off for disabling or shutting off the generator 18 directly.Line 64 could, if desired, be a mechanical or pneumatic linkage orswitch if desired.

Regarding the operation of the foot pedal 62, microprocessor 20 will beprogrammed such that if the foot pedal 62 is depressed while thegenerator 18 is inactive but ready for operation, the therapy procedureshall begin. Releasing the foot pedal within a predetermined period oftime, say three (3) seconds, will terminate the therapy session. Keepingthe foot pedal 62 depressed for the predetermined period of time, suchas three seconds, will cause the control to latch and the therapy willcontinue even if the pedal is released. Pressing on the foot pedal 62when the generator 18 is operating will terminate the therapy session.As noted, any surgical instrument 26 may also include an on/off button46 that operates in the same manner as the foot pedal 62.

VETAD 10 may also include visual indicators for indicating variousoperating conditions thereof. For example, VETAD 10 may include one ormore light emitting diodes (LEDs) 66 (FIG. 2) that communicate withmicroprocessor 20 over a line 68. In one embodiment of the presentinvention three such LEDs may be used. For example, an LED 66 a may beused to indicate that power is being supplied to the apparatus 10; andLED 66 b may be used to indicate that RF power is being generated bygenerator 18; and an LED 66 c may be used to indicate a system error orfailure of some kind. An auditory indicator of system operation may alsobe provided by a “beep” alarm 70 that communicates with microprocessor20 over a line 72. Beeper 70 may sound for example, during generation ofRF power by generator 18 or to warn of an error condition.

Referring still primarily to FIG. 3, it will be observed that thesurgical instrument 26 may include one or more thermocouples 74 thatcommunicate with a temperature measurement circuit 76 of VETAD 10 over aline 78. In addition VETAD 10 may include one or more auxiliarythermocouples 80 that are placed at selected tissue locations as will beexplained further below. Thermocouples 80 communicate with temperaturemeasurement circuit 76 over a line 82. Circuit 76 in turn communicateswith microprocessor 20 over a line 84. Thermocouples 74 and 80 areprovided for providing temperature measurements at selected tissuelocations to indicate the progress of the ablation therapy. That is, bymonitoring the temperature of the tissue, it can be determined if thetissue is being ablated and whether healthy tissue whose ablation is notdesired is being affected. Circuit 76 may take the form of a singlecircuit or it could comprise two identical circuits such that theapparatus 10 provides a redundant safety feature. With a double circuitthe thermocouples would be divided into two groups of thermocouples withone group providing temperature indicating signals to one circuit andthe other group providing temperature indicating signals to the othercircuit. With a dual circuit structure if one circuit failed the otherwill remain operational and the therapy will continue. Shutdown of thetherapy and the, apparatus 10 would occur with failure of both of thedual circuits of the temperature measurement circuit 76. Statedotherwise, a double circuit would provide an additional patient safetyfeature. For example, a surgical instrument 26 could comprise a straightor helical needle with two thermocouples or temperature sensors thereon.Both thermocouples would provide signals indicative of temperature tothe circuit 76 and would output both sets to the microprocessor 20. Ifeither set of signals indicated a therapy failure or discontinuancestate, such as one of them reaching and exceeding the primarytemperature threshold, the application of RF power would bediscontinued. In this way, one circuit could fail and provide falsereadings of low temperatures, but patient safety would be maintained bythe second circuit providing accurate temperature signals to themicroprocessor 20.

It will be understood line 86 illustrates the engagement of the surgicalinstrument 26 with the patient 28 and that the line 88 illustrates theengagement of the auxiliary thermocouples 80 with the patient 28. Itwill further be understood that the lines 30, 44, 48, and 78 extendingbetween the instrument 28 and the VETAD 10 will typically be supplied bya single connection 91 (FIG. 2) with VETAD 10.

The generator 18 will include an impedance measurement circuit 90 thatcommunicates with microprocessor 20 over a line 92 and an arc detectioncircuit 94 that communicates with microprocessor 20 over a line 95.Generator 18 will also include the RF power generation circuit 96, whichcommunicates with microprocessor 20 over a line 97. Preferably,generator 18 will also include a circuit 98 to monitor the returnelectrode—the ground pad 38. In one embodiment of the present invention,ground pad 38 will comprise a split electrode. Circuit 98 will monitorthe impedance between the split electrodes to reduce the likelihood ofpatient injury due to a high power flow through only one of theelectrodes. It should be understood that although the present inventionis shown as having generator 18 include the impedance measurementcircuit 90, the arc detection circuit 92, and the RF power generationcircuit 96, circuits 90 and 92 could be partially hardware and partiallysoftware running in microprocessor 20.

Referring to FIG. 1, it will be understood that VETAD 10 will includethe appropriate data output port, such as an RS-232 port to enable theattachment of external devices, such as a computer 99 to provide fordownloading data in real time or at a later date and/or permanentstorage of data gathered during an ablation therapy session. It will beunderstood that computer 99 may be remotely located from VETAD 10 andcommunicate therewith over a telephone or other data transfer line.

Referring now to FIGS. 2 and 4, the various user interface screens thatmay be displayed in display 22 is shown. Display 22 may include anelectroluminscent display or a liquid crystal diode (LCD) or similardisplay. Display 22 further includes a rotary knob/encoder 100. In usinga display 22, the rotary encoder will be rotated to change the displaybetween the pump, setup, procedure, and system to display predeterminedparameters relating to operation of the apparatus 10. The display/userinterface will enable the operator to select and adjust parameters for aspecific tissue ablation, purge the fluid lines, download or clear logfiles of an ablation procedure(s), adjust fluid flow rates and RF powerlevels, adjust maximum temperatures for the various monitoredtemperatures, etc.

At startup, a language selection screen 102 will appear on the main menubar 104 to allow the operator to select a particular language for useduring the procedure. The knob 100 can be rotated to the appropriatelanguage and then pushed to select a language. Following the selectionof a language, other user interface screens can be selectivelydisplayed. Such screens may include a purge screen 106, a setup screen108, an ablation procedure start screen 110, a pre-infusion flow screen112, an ablation procedure screen (power on) 114, an ablation procedurescreen 116 (power off), and a system screen 118. During the display ofthe various user interface screens the operator will be able to selectthe various operation parameters, such as power level, infusion rate,temperature thresholds, pre-ablation infusion rates and times, and newplacements or restarts of the procedure with the same placement of theinstrument to name a few. In addition, an embodiment of the presentinvention may also allow the operator to specify the type of tissuebeing ablated. During an ablation procedure the display 22 will show thetemperatures measured by the thermocouples 74 and 80, the infusion rate,the amount of conductive fluid remaining in the supply the power level,the impedance, and the unique identifier of the particular instrument 28if there is one.

Display 22 will also preferably indicate a plurality of levels of alarmconditions, for example, for a warning state, a therapy failure state,and a system failure state, as indicated by light emitting diode 66.Preferably, each alarm condition will be made known by the lighting ofthe particular associated LED and a distinctive sound or tone producedby an appropriate speaker 70 (FIG. 3) in response to a signaltransmitted from microprocessor 20 via line 72. LED 66 will lightindicating a warning state according to certain predetermined parametersmonitored by microprocessor 20. A warning state would indicate thatcertain operating parameters are being exceeded that could be ofconcern, but that do not warrant terminating the therapeutic procedurebeing undertaken. Preferably simultaneously with a lighting of LED 66and the sounding of the distinctive tone, a message appropriate to theparticular warning signal would be displayed in display 22. For example,certain predetermined warning events as shown in Table 1, below, may beprogrammed to trigger the lighting of LED 66 and the display of thewarning message corresponding to the event triggering the warning state.Table 1 is meant to be exemplary only and not limiting of the presentinvention.

TABLE 1 Event Warning Triggering Monitored Message Warning StateCondition Failure Mode Action Displayed 1. Conductive Conductive Volume= 0 None Conductive Fluid Low Fluid Volume Fluid Low 2. ImpedanceImpedance Impedance None Impedance Approaching within 25% of approachingMax/Min Limit the maximum/ limit minimum limit 3. Approaching Timeelapsed 1 minute to end None 1 minute end of therapy of maximumremaining therapy time

It will be understood that other warning triggering events could also beused to generate a warning message and that the present invention is notlimited to those listed here.

When a therapy failure state occurs, LED 66 may light and speaker 70 maysound the distinctive tone appropriate for this state. This state willoccur when a therapy session has ended prematurely, for example, as aresult of exceeding the maximum impedance. When a therapy failure eventoccurs, the VETAD 10 will remain functional and a new therapy can beprogrammed and begun. As with the warning state, a warning message willbe displayed in display area 22. For example, certain predeterminedwarning events as shown in Table 2, below, may be programmed to triggerthe lighting of LED 66 and the display of the warning messagecorresponding to the event triggering the therapy failure state.

TABLE 2 Event triggering Warning therapy failure Monitored Message stateCondition Failure Mode Action Displayed 1. Impedance ImpedanceAbove/below End Impedance out of range the maximum/ Therapy High/Lowminimum impedance 2. Conductive Conductive Volume = 0 End Conductivefluid exhausted fluid volume Therapy fluid out 3. Arc detected Presenceof an Exceed the End Arc detected arc predetermined Therapy number ofallowed arcs 4. Temperature All active Temperature End Temperature toohigh thermocouples exceeds Therapy too high or thermistor hardwiredsafety limit 5. Maximum Time elapsed Maximum End Maximum therapy timertherapy time Therapy therapy time reached reached

It will be understood that other therapy failure triggering events couldalso be used to generate a therapy failure warning message and that thepresent invention is not limited to hose listed here.

When a system failure condition occurs, the system failure LED 66 willbe lit, the appropriate tone sounded, and corresponding warning messagedisplayed. For example, certain predetermined warning events as shown inTable 3, below, may be programmed to trigger the lighting of LED 66 andthe display of the warning message corresponding to the event triggeringthe system failure state.

TABLE 3 Event triggering Monitored Message Warning failure stateCondition Failure Mode Action Displayed 1. Self test fails Allparameters Any failure Inhibit all System failure- operation maintenancerequired 2. Software Watchdog timer Timer expires Inhibit all Systemfailure- watchdog timer operation maintenance required 3. BrokenThermocouple Open End Faulty thermocouple continuity thermocoupleTherapy thermocouple

It will be understood that other system failure triggering events couldalso be used to generate a warning message and that the presentinvention is not limited to those listed here.

Alternatively, where a display such as display 22 is used, the warningcondition may be signaled by an appropriate warning light appearing onthe display rather than a separate LED.

VETAD 10 may utilize a primary thermocouple in its operation and mayutilize one or more secondary thermocouples to monitor the temperatureof the tissue being ablated and the surrounding tissue temperature.Thus, the primary thermocouple may be placed within or near the area ofdesired tissue ablation and the secondary thermocouples may be placedeither at the desired perimeter of the lesion to be created or within anarea of tissue outside of the tissue to be ablated or both. Moregenerally, the secondary thermocouples will be placed wherever it isdesired to measure tissue temperature. With some surgical instruments,the thermocouples may be included with the surgical instrument ratherthan provided for separately. VETAD 10 and the operation of the pump 16and generator 18 can therefore be controlled based upon measuredtemperatures, as will be described further below.

VETAD 10 will also provide an impedance measurement 90 between thevirtual electrode and the tissue being treated. As noted, it is desiredto maintain the impedance between about ten (10) ohms and about fivehundred (500) ohms. Maintaining the impedance within this general rangewill allow the continuous application of radio frequency power to thepatient and thus allow the ablation procedure to continue. Failure tomaintain the impedance within this general range will allow the currentdensity or voltage amplitude to increase locally, which may in turncause steam bubbles to form and desiccation of the tissue, damage to thedisposable surgical instrument, the generator 18, or a combinationthereof. Desiccation, in turn, may allow arcing between the electrodeand the tissue. As the impedance rises, the VETAD 10 will reduce theamount of applied radio frequency power according to predeterminedcriteria to be discussed below. Generally, the conductive fluid flowrate will not be diminished, however, resulting in the continued flow offluid away from the surgical instrument. This continued flow recouplesthe radio frequency energy to the tissue, enlarges the virtualelectrode, reduces the current density, and thus reduces the impedance.

VETAD 10 will also preferably include an arc detection circuit 92. Inaddition to occurring because of the desiccation of tissue mentionedpreviously, arcs may occur as a result of an air bubble or other failureto deliver an adequate timed flow of the conductive fluid or aninadequate fluid flow rate for the level of radio frequency powerapplied to the tissue. Thus, as will be described further below, the arcdetection circuit 92 will shut off the applied radio frequency powerwhen a predetermined number of arcs are detected within a predeterminedtime period.

The total ablation time and ablation intervals will also preferably becontrolled by VETAD 10. Thus, VETAD 10 may include a clock circuit 120that communicates with microprocessor 20 over a line 122. It isdesirable to control the total ablation time to facilitate patientsafety and prevent the application of radio frequency power to thepatient beyond that maximum time period. Thus, VETAD 10 will include aclock for recording the total time that ablative radio frequency energyis applied to the patient. When the maximum time limit is achieved,which may be predetermined by the manufacturer and thus not subject tooperator control or left to the discretion of the operator to input as aparameter for a particular procedure, VETAD 10 will automatically ceaseto apply radio frequency power to the patient. If desired in aparticular embodiment of the present invention, once the VETAD 10 hasreached the maximum ablation time, an ablation interval timer may beactivated to prevent beginning anew the application of radio frequencypower to the patient. The interval timer will prohibit reactivation ofthe generator 18 until a predetermined time period has expired, thusallowing the generator 18 to cool and to prevent the continuous,uninterrupted application of power to the patient.

Yet another control function of the VETAD 10 will be to control thepre-ablation infusion of the conductive fluid into the tissue to beablated. The period of pre-ablation infusion can be either determined bythe user or defaulted based upon the particular disposable surgicalinstrument 26 to be utilized. Pre-ablation infusion will allow theinterstitial permeation of the conductive fluid and the creation of thevirtual electrode. The pre-ablation infusion period may be shortenedfrom the full period with the same or different rates of infusion whereanother ablation procedure will be performed with the same instrumentplacement or may be set to a full pre-infusion period at the full flowrate for new instrument placements.

It will be understood that upon start up of VETAD 10 that certain selfchecks will be made. Once the ablation therapy procedure is begun, thepre-ablation infusion will begin and extend for a pre-determined timeand at a predetermined flow rate. In one embodiment of the presentinvention, following the pre-ablation infusion, an impedance check willbe made by applying a predetermined power level to the tissue andmeasuring the impedance. If the impedance falls within a predeterminedrange set by either the operator or the surgical instrument 26, then theablation procedure will begin. If the impedance does not fall within thepredetermined range, the pre-ablation infusion will be continued for apredetermined, extended time period, by way of example, a time periodequal to at least twenty percent (20%) of the original pre-ablationinfusion time period, and another impedance check will then be taken.This pre-ablation infusion/impedance check cycle will be repeated apredetermined number of times, at least once and preferably three times.If the impedance does not fall within the specified range at the end ofthe predetermined number of infusion/impedance check cycles, then VETAD10 will discontinue the ablation procedure.

The VETAD 10 will be operable in either an automatic or auto mode and amanual mode. In automatic mode, control of the ablation procedure andthus of VETAD 10 will advantageously utilize at least one of severalmethods or control loops including a primary and secondary temperaturecontrol loop, impedance control of radio frequency power loop, andimpedance control of conductive fluid flow loop. Initially, upon startupof the VETAD 10, the VETAD 10 will be in an automatic mode of operationand will operate according to default parameters and control functionsfor the temperature thresholds, fluid flow rate, and radio frequencypower levels. It should be understood, however, that the disposablesurgical instrument 26 may indicate that the apparatus 10 is to operatein a manual mode upon start up. These parameters will be controlledbased upon either operator inputted parameters such as tissue type anddesired lesion size or upon the recognition by the microprocessor 20 ofthe particular type of surgical instrument based upon the pin connectionor the like between the surgical instrument 26 and the VETAD 10 or uponthe default parameters supplied to VETAD 10 by the memory 42 of theinstrument 26. When in the automatic mode of operation, the operator ofthe VETAD 10 will still be able to change certain predeterminedoperational parameters, including the maximum power level and the timeto maintain the predetermined primary and secondary temperatures.

An operator may determine that a manual mode of operation is preferableto the automatic mode of operation. In the manual mode, certain of thecontrol loops, such as the primary temperature control loop, will bedisabled and the generator 18 will operate at a constant power output.The impedance control of radio frequency power will remain activated toprovide a safety margin for the patient. In addition, the microprocessor20 will terminate the application of radio frequency power if either theprimary or any secondary temperature maximum is exceeded. When in themanual mode of operation the operator will be able to control the radiofrequency power level, which shall remain constant unless changed by theoperator, the primary and secondary temperature thresholds, theconductive fluid flow rate, the pre-ablation infusion rate, and thepre-ablation infusion time period.

To summarize the foregoing, the present invention will provide apparatusand method for therapeutically ablating tissue in response to a varietyof measured parameters. Generally, the ablation process must becontrolled in response to a variety of parameters due to the variationsin the thermodynamic, electrical conductivity, and fluid dynamiccharacteristics of the tissue being ablated. In response to the variousmeasured parameters, the present invention will control the virtualelectrode and thus the ablation process by varying the applied power andfluid flow. Referring to FIG. 5, it can be seen that the inputparameters may include the temperature as measured by a primarytemperature measuring device, such as a thermocouple, one or moresecondary temperatures as measured by a secondary temperature measuringdevice, the impedance between the virtual electrode and the groundplate, the detection of arcs, and the time of the application of the RFcurrent or power. The control loops provided by the present inventionwill control the applied power and the conductive fluid flow in responseto one or more of those measured parameters.

The various control loops contemplated by FIG. 5 are shown in Table 4below.

TABLE 4 Controlled Control Loop Name Parameter Inputs TemperatureControl Loop RF Power Primary Temperature Secondary Temperatures Max RFPower Time Impedance Out-of-Range RF Power Tissue-Virtual ElectrodeControl Loop Impedance Time Arc Detection Control Loop RF Power ArcDetection (Impedance) Time Fluid Flow Control Fluid Flow Impedance

As seen in FIG. 5 and Table 4, there are three control loops thatfunction to control the level of applied RF power and one to control therate of fluid flow. These various control loops will interact with eachother to control the ablation process through the present invention.This interaction is shown generally in FIG. 6. The first control loop isthe Arc Detection Control Loop 120. An arc can occur if the temperatureof the tissue being ablated gets too high and results in desiccation orif an air bubble passes through the surgical instrument into or on thetissue being ablated. This loop will be described in more detail below,but suffice it to say that if an arc is detected the RF current will bediscontinued for a predetermined period of time, such as two seconds forexample, as at 122 while fluid flow is maintained into or on the tissue.If a predetermined number of arcs are detected in a predetermined timeperiod, such as three or four arcs within one minute, then the ablationprocedure will be discontinued as at 114 and an error will be indicated.Preferably, the Arc Detection Control loop will always be running in thebackground.

Generally, there may be minimum and maximum tissue-virtual electrodeimpedance values and slope, that is, rate of change, ranges establishedfor each ablation procedure so as to provide for a safe and effectiveablation will be established. These values may be established as adefault held in the microprocessor 20 in VETAD 10 or the values may beheld in memory 42 positioned on the instrument 26, which aresubsequently provided to VETAD 10. When the measured impedance isgreater that the maximum level of slope established as the thresholdthen the RF power will be reduced as at 128. Alternatively, if themeasured impedance is within the ranges established, then the ImpedanceControl Loop 126 will increase the applied RF power to a new maximum.

The Arc Detection and Impedance Control Loops are used to monitor thevirtual electrode and the ablation procedure and to modify the RF powerlevel if necessary. If such modifications to the applied power leverwere not made the virtual electrode may become ineffective at ablatingthe tissue, thus making the ablation procedure an unsuccessful one. Forexample, tissue desiccation could occur, thus preventing the RF energyfrom effectively and safely coupling to the tissue.

When the measured impedance is determined to be within the desiredcriteria and stable, control of the RF power will default to theTemperature Control Loop 132. The desired impedance criteria may eitherbe a predetermined range of operation, a threshold, a rate of impedancechange or the like. The Temperature Control Loop 132 will control theapplication of RF current to the tissue if the Arc Detect Loop 120 andImpedance Control Loop 126 are satisfied. Preferably, since tissuetemperature determines whether cell death occurs, it is desirable forTemperature Control Loop 132 to maintain control of the ablationprocedure throughout the entire ablation process. Generally, theTemperature Control Loop 132 will always be monitoring a primarytemperature during an ablation procedure and will preferably also bemonitoring at least one secondary temperature. The primary temperaturewill typically be substantially near the center or edge of the lesion tobe created while the secondary temperatures will be spaced therefrom atpredetermined locations. Once again, maximum values for both the primaryand secondary temperatures will be established and held either in memoryin VETAD 10 or will be supplied to VETAD 10 and thus the microprocessorby a microchip held on instrument 26. If the primary measuredtemperature is below the maximum temperature established for theparticular ablation procedure, then the RF current will be increased tothe maximum as at 134. If the primary measured temperature is above themaximum temperature established for the particular ablation procedure,then the RF current will be appropriately reduced, discontinued ormodulated as at 136 and as discussed further below. If the temperaturesmeasured by the secondary measuring devices exceeds the secondarythreshold, then the RF power will be shut off as at 138. Finally, theTemperature Control Loop will discontinue the application of RF powerwhen the temperature of the primary thermocouple exceeds the primarythermocouple maintain time (which may be from zero to five minutes, witha typical nominal range of about thirty to about sixty seconds) as at140. In sum, in the automatic mode of operation the Temperature ControlLoop 132 will control the application of the RF current to the tissuebased upon the temperature measured by the primary temperature measuringdevice and, if present, upon the temperature measured by the secondarytemperature measuring device.

The primary temperature is intended to be the input which controls theablation procedure and the resulting lesion size. This temperature maybe measured by a thermocouple, which may be placed by hand by theclinician. By way of example only, a physician may dispose the primarythermocouple at the outer edge of a tumor. Once the temperature asmeasured by this thermocouple reaches a predetermined temperature for apredetermined time period of a sufficient duration to ensure the tumoris fully ablated, the procedure will be discontinued. The application ofthe RF current via the virtual electrode will result in a temperaturegradient expanding outwardly from substantially the center of thevirtual electrode. The positioning of the primary thermocouple will usethe knowledge of the temperature values and temperature rates of changeto set the primary thermocouple threshold and maintenance time to createthe desired lesion size. It should be understood that more than oneprimary temperature sensor could be used in accord with the presentinvention. For example, where the surgical instrument 26 takes the formof a forceps that is used to resect a selected portion of tissue, suchas lung or liver, the instrument 26 will ablate tissue along apreselected path. It may be desired to monitor the temperature at morethan one location along this resection path. In such a case, instruments26 would be include a plurality of “primary thermocouples” such that thetemperature of the tissue being ablated can be monitored at multiplelocations.

In general, the Temperature Control Loop 132 will set the RF power to amaximum allowed level until the Primary Temperature threshold isreached. Once the Primary Temperature threshold is reached, theTemperature Control Loop 132 will modulate the RF power such that thetemperature threshold is maintained for the predetermined period oftime. The time over which the temperature threshold is maintained isspecified either by the disposable table held in the memory chip of theinstrument 26 or the physician. After the specified time the RF power isshut off and the ablation procedure is successfully complete. Themaximum RF power may be established by the physician, a microchip 42 inthe surgical instrument 26, or based upon feedback received from theImpedance Control Loop. Similarly, the Primary Temperature threshold maybe set by the physician, by the microprocessor in VETAD 10, or byinstrument memory 42 forming part of instrument 26.

The Secondary Temperatures, which may also be measured withthermocouples, may be used by the physician to protect tissues andorgans near the ablation/lesion area from reaching damagingtemperatures. If any one of the Secondary Thermocouples exceeds aspecified threshold, then the RF power will be shut off. Again, theSecondary Thermocouple thresholds can be set by the instrument memory 42or the physician.

Alternatively, the Secondary Thermocouples may be used to ensure thatthe outer edges of the desired lesion reach adequate temperatures Inthis implementation the RF power would not be shut off until all of thedesired thermocouples (Primary and Secondary) reach the desiredtemperature. Additionally, the Secondary Thermocouples can be positionedin the desired lesion area to show the temperature gradients as thetemperature expands from the center of the lesion. This information—thetemperature gradients—can show both that an adequate lesion has beencreated or that healthy tissue was protected from being ablated.

Referring now to FIG. 7, the various inputs that can be used to controlRF power as well as the various thresholds and parameters can beobserved. Thus, for example, the various inputs to the Arc DetectionControl Loop 120 will include the impedance measurements and time. Thesemeasurements will be compared with parameters that include a counter,total number of arcs counted, the measured impedance of an arc, and thetime frame during which successive arcs occur. The inputs to theImpedance Control Loop 126 will include the impedance measurements andtime, and if desired, the rate of change in the impedance. Thesemeasurements will be compared to the impedance threshold and if desireda rate of change threshold. The control loop will then operate tocontrol the apparatus 10 by varying the applied power in response toparameters such as a power reduction percentage, time during which poweris reduced, and the default maximum power for the procedure. TheTemperature Control Loop 132 may include as inputs the maximum power,the temperature measurements provided by the primary and secondarythermocouples, and the time. These inputs will be utilized by thecontrol loop 132 and compared against the voltage clamp level, thethresholds for the temperature measurements, the time for which thetemperature measurements should be maintained to assure the desired celldeath, the temperature overshoot above the threshold allowed beforeshutdown occurs, and the maximum ablation time. It will be understoodthat the foregoing inputs, thresholds, and parameters are by way ofexample and that other inputs, thresholds, and parameters can beutilized with the present invention.

The Fluid Flow Control Loop 150 is shown generally in FIG. 8. Fluid FlowControl Loop 150 may utilize impedance measurements to modify the fluidflow rate. Thus, where the impedance measurement is trending upwardly atan excessive rate or reaches a predetermined level, the fluid flow canbe increased to increase the size of the virtual electrode and thusspread the applied power throughout an increased volume of tissue. Theincreased size of the virtual electrode decreases the current or energydensity and thus reduces impedance. The fluid flow rate can becontrolled in several ways, such as linearly increasing the fluid flowin response to the measured impedance, providing a step wise change inthe fluid flow in response to the measured impedance, or providing alook-up table that provides the desired rate of fluid flow in responseto the measured impedance. Generally, the surgical instrument willprovide several default parameters to the VETAD 10, such as the defaultfluid flow rate, the impedance at the default fluid flow rate, the rateof change in impedance to the change in impedance flow rate—the slope—,the maximum fluid flow rate, and the minimum fluid flow rate.

By way of example only, with a linear relationship established betweenthe impedance and the rate of fluid flow, the default fluid flow ratecould be set at 1.4 cubic centimeters per minute; the impedance at thedefault fluid flow rate at 100 ohms; the change in impedance to changein fluid flow rate at 0.1 cubic centimeter per minute per 10 ohms changein impedance; the maximum fluid flow rate at 2.5 cubic centimeters perminute; and the minimum fluid flow rate at 0.8 cubic centimeters perminute. Thus, the flow rate would initially be established at 1.4 cubiccentimeters for a default impedance of 100 ohms. For each change inimpedance of ten ohms then the flow rate would be correspondinglyincreased or decrease until the maximum or minimum fluid flow wasreached. Such a linear relationship as just described can be graphed asshown below in FIG. 21. Again, it will be understood that this exampleis exemplary only.

FIG. 22 below illustrates by way of example only a step wise approach tocontrolling fluid flow based upon impedance measurements. Generally,such an approach may define initial parameters including: nominal fluidflow rate of 0.5 cubic centimeters per minute, a medium fluid flow rateof 1.0 cubic centimeters per minute, a high fluid flow rate of 2.0 cubiccentimeters per minute, a medium impedance threshold of 100 ohms and ahigh impedance threshold of 125 ohms. Generally a fluid flow rate willbe, established for impedance measurements below a first level and asecond fluid flow rate will be established for impedance measurementsabove the first level. Additionally fluid flow rates can be set foradditional impedance measurements. Thus, in the example given and shownbelow, three flow rates are established based upon the measuredimpedance as compared with the fluid flow impedance thresholds.Operationally, fluid flow rates should be maintained for a predeterminedperiod of time, such as about one second to about five seconds, beforedecreasing the flow rate due to a subsequent impedance measurement.

As noted above, flow rate could also be controlled based upon a varietyof parameters, including impedance, temperature, and RF power level.Quadratic or higher order equations could be devised to control the flowrate that would increase or decrease the flow rate as desired and at therate desired.

FIG. 9 illustrates the microprocessor 20 and its function generally.Microprocessor 20 will receive as inputs the various temperaturemeasurements, the measured impedances, and detected arcs. It will outputsignals to control the flow rate and the RF power. Signals indicative oftemperature will be compared against temperature thresholds and when theprimary temperature reaches the maximum temperature to a temperaturemaintain timer to monitor the length of time at which the tissue beingablated is held at the maintenance temperature. This information willalso be provided to a timer control loop, illustrated as being a tenminute timer for purposes of illustration. The measured impedances willbe provided as a signal input into the fluid flow control loop and thesoft arc control loop. The fluid flow control loop—the impedance controlof fluid—will provide a flow rate to the pump 16 as an output of themicroprocessor 20. In addition, inputs will determine whether theinitiation of RF power is a new placement or a restart with the surgicalinstrument 26 at the same site. Where a new placement is made, thepre-ablation infusion rate will be for the full specified time periodwhereas when the initiation is a restart, a shorter period of infusionwill be used.

As noted, the impedance measurement signals will be provided to themicroprocessor 20 and processed by the soft arc—the impedance control ofRF power—loop. Another input to that loop will be time. When an arc isdetected the RF power will be discontinued for a predetermined period oftime. Detected arc signals will be provided to the “hard arc” loop whichwill determine whether the predetermined number of arcs occur within thepredetermined time period. If so, the RF power will be discontinued. Inaddition, a timer which determines how long RF power has been appliedwill discontinue the application of power upon the expiration of apredetermined time period, such as ten minutes for example.

The various control functions of the VETAD 10 having been generallydescribed above, specific details of the operation of the apparatus 10and the control methods or loops will be discussed below with referenceto the figures. Referring to FIG. 10, the overall operation of theapparatus 10 will first be described generally. Thus, followingplacement of the power source at the site of the desired ablation, atthe start of the apparatus 10 as indicated at 200 a pre-ablationinfusion of conductive fluid may be made and an impedance check at lowpower duration, say one watt-second, may be made as indicated at 202. Ifthe impedance check is satisfactory, meaning that the measured impedanceis within the predetermined limits of about 10 ohms to about 500 ohms,or some other threshold provided to the VETAD 10 by the instrument 26 orthe operator, then the ablation procedure will begin and will becontrolled by the various described control loops as at 204. Theimpedance checks may also look at the difference between the initialimpedance measurement and subsequent re-checks of it.

The ablation procedure will then either be successfully completed as at206 or will be halted for a predetermined reason, such as by exceedingmaximum secondary temperature limits or detection of a predeterminednumber of arcs as indicated at 208. In addition, the ablation procedurecan be discontinued manually by use of the stop button 54 on theapparatus 10 or the foot pedal 62. Even where the operating parametershave been determined by the surgeon, the control loops will operate tohalt the procedure when certain critical parameters, such as lowimpedance or high impedance are reached. For example, a low impedanceshut off is desirable since the lower the impedance the greater thecurrent that is being routed from the apparatus 10 through the patientand back to the apparatus 10, thereby jeopardizing the electronics ofthe apparatus 10 and the surgical instrument 26. Alternatively, a highimpedance shut down is desirable since that indicates that the tissueclosely adjacent the power delivery electrode is desiccating and thusinterfering with the distribution of applied power throughout the extentof the virtual electrode. Following either the discontinuation of anablation procedure or the successful completion of a procedure, theablation procedure may be restarted or the instrument 26 may be placedanew at another location as indicated at 210 and the procedure startedanew. Where a restart occurs, that is, with the same needle placement,the pre-infusion of electrolytic fluid may be selected for the full timeat the full flow rate or a reduced time and or a reduced flow rate. Thereduced time and flow rate would reduce the likelihood of an occurrenceof an excessive fluid delivery to the tissue.

Referring now to FIG. 11, a start-up operating routine will be describedin greater detail. Thus, at the start of the procedure, it will bedetermined whether the procedure is a restart at an existing placementor a new placement as at 220. Where a restart is indicated, fluid flowwill be preferably initiated for a predetermined period of time, whichcan be anywhere from about one second to about sixty seconds, asindicated at 222. An impedance check will be made at low power duration,such as about one watt-second but generally 15 watt-seconds or less, asindicated at 224, to determine the initial impedance levels. Theimpedance check could be made at full power levels, however. Themeasured impedance will then be compared with a threshold impedancelevel at 226. If the measured impedance level is below a predeterminedthreshold, then the RF power control loops will be initiated and theablation procedure will start, as at 228. If the threshold is exceededhowever, then an error will be indicated as at 230 and the procedurewill be halted.

Where the start up is for a new placement, that is, a new site for anablation, a baseline check will be made to initially check the impedanceas at 232. Once again, the power level utilized to make this initialimpedance check should be minimized to minimize the risk of arcing.Fluid flow will then be initiated for a predetermined period of time andrate as provided by the instrument 26 (or the operator when working inmanual mode) as at 234. Once the predetermined time period has elapsed,another impedance check will be made as at 236. A calculation will beperformed to determine the percentage change in the impedance at 238.Thus, the impedance measured at 236 will be subtracted from thatmeasured at 232, the product will be divided from the initial baselinemeasurement and then multiplied by 100 to provide a percentageindication of the impedance change following fluid flow at 234. Thepercentage change will then be compared to a pre-determined level, suchas 25% as indicated in the FIG. 11 at 240. Where the impedance change isless than the threshold, then an error will be indicated as at 242 andthe procedure will be halted. If the percentage change is greater thanthe predetermined percentage change amount, then the measured impedancewill be compared with the predetermined, pre-ablation threshold as at244. If the impedance measurement is less than the pre-ablationimpedance threshold then the RF power will be started and the variouscontrol loops will be operational as at 246. Where the final impedancemeasured at 236 is greater than the threshold impedance level at 244 theprocedure will be halted an error signal will be given as at 242.

The routine illustrated in FIG. 11 is intended to safeguard both thepatient and the apparatus 10. Where the impedance checks indicate thatthe impedance is too high, that is above the threshold, then the risk ofarcing increases. Finally, where the impedance fails to drop by apredetermined percentage, then there is an indication that the initialflow of conductive fluid has failed to sufficiently disperse within orupon the tissue to provide for a safe ablation of the desired tissue.The aforementioned predetermined percentage may be in the range of 0% to100% of the initial impedance measurement. Setting the predeterminedpercentage to 0% has the effect of disabling the feature.

Table 5 provides an example of the various parameters that can beutilized to control the apparatus 10 in a pre-infusion routine similarto that illustrated in FIG. 11 and the typical values associated withthose parameters.

TABLE 5 TYPICAL PARAMETER VALUE LOCATION DESCRIPTION Flow Rate 0.5cc/min Surgical Flow rate for pre-ablate Instrument infusion. If zero,then skip pre- ablate infusion Pre-ablation Time for New 30 sec SurgicalLength of time for initial pre- Placement (Full Pre- Instrument ablateinfusion ablation infusion) Pre-ablate Impedance 25% Surgical Minimumamount impedance Change Instrument must fall from pre-infusion baselineto post infusion measurement for ablation to continue Pre-ablateImpedance 75 ohms Surgical If impedance below threshold, ThresholdInstrument start ablation with out regard to success of Pre-ablateImpedance Change Restart Pre-ablation Flow 5 sec Surgical Time to infusebefore RF On in Time for Same Placements Instrument/ case of restartedor continued (Short Pre-ablation Table in ablation infusion) memory

Referring now to FIG. 12, the Arc Detection Loop 120 will be explainedmore fully. The impedance measurements provided by apparatus 10 will becompared with a minimum impedance threshold at 260. As illustrated, theminimum has been indicated at 25 ohms. This minimum could be set higheror lower as desired. Low impedance measurements indicate, however, thatthe power generated by RF generator 18 contains high current levels.Consequently, to avoid damage to the apparatus 10 and the instrument 26,when the measured impedance falls below the minimum, the power will beshut off as indicated at 262.

If the impedance measurements exceed the minimum level, the Loop 120will monitor the impedance measurements to determine whether arcing isoccurring at 264. Arcing is indicated by high impedance measurements.For purposes of illustration, an arc is defined as an impedancemeasurement exceeding a predetermined level for a predetermined periodof time, here, 500 ohms for 500 milliseconds. Within the limits of theelectronics, shorter or lesser time periods can be defined for the arcdetection. If an arc meeting the predetermined criteria is detected,then the total number of arcs will be determined. When the total numberof arcs exceeds a predetermined number within a predetermined timeperiod as at 266, then the power will be shut off as at 262. Asillustrated, three arcs occurring within a minute will result in powerbeing shut off and the procedure discontinued. It should be understoodthat a different number of arcs, such as two, four, or some greaternumber could also be used consistent with patient safety and the safeoperation of the apparatus 10. If an arc is detected, but the criteriafor a complete shut down of power is not met, the power will be shut offfor a predetermined period of time, here two seconds as at 268. Fluidflow will be continued during this time period, allowing the conductivefluid to spread throughout the desired tissue ablation site. After twoseconds, or such other period as is specified, power will again beapplied to the patient through the virtual electrode. Where theTemperature Control Loop is maintaining temperature or where thegenerator is being operated manually, then a certain amount of time willbe added to the maintain time timer to make up for the period of timeover which the RF power was off. Where an arc is not detected thencontrol of the ablation procedure and thus the RF power will be turnedover to first the Impedance Control Loop 126 and then the TemperatureControl Loop 132. It should be noted that these control loops couldoperate simultaneously, but in the order of precedence shown in FIG. 7.That is, each control loop could operate at all times, with controlprecedence of the applied RF power and fluid flow being In addition,after a power shut off as at 268, control will be turned over to firstthe Impedance Control Loop 126 and then the Temperature Control Loop132.

Table 6 below indicates the various parameters and typical value thatcan be utilized by the Arc Control Loop 120. It will be understood thatthese parameter values can take on other values and that the presentinvention is not limited to these parameters.

TABLE 6 TYPICAL PARAMETER VALUE LOCATION DESCRIPTION Number of 3 TableNo. of arcs before ablation Arcs ends (over Arc Detect Counter Time) ArcImpedance 500 ohms Table/Hard- Impedance threshold at Ware which arc isdeclared Time for Arc 500 msec Table/Hard- Max time period in which Warearc must be detected Arc Detect 1 minute Table Period over which arcsCounter Time are to be counted towards “Number of Arcs” limit Arc DetectRF 2 sec Table Length of time RF power Off Time is off after arcdetected Impedance 25 ohms Table/Surgical Minimum impedance LowThreshold Instrument threshold lower than this and ablation ends

Referring now to FIG. 13, an arc detection control loop 300 will now beshown as described. Thus, loop 300 will detect and count the number ofarcs that occur. If a predetermined number of arcs, say three, occurswithin a predetermined time limit, say one minute, then the radiofrequency power will be turned off, Thus, VETAD 10 will detect arcs asat 302, count the number of arcs occurring within a time period ΔT as at304, and compare the number detected at 306 with the allowed maximumnumber of arcs occurring within a predefined time interval held inmemory as at 308. If the maximum limit is exceeded, then the therapywill terminate as at 310. If, however, the maximum is not exceeded, thenthe RF power will be discontinued for a predetermined period of time asat 312. Where control of the ablation process requires that thetemperature be maintained for a predetermined period of time, then apredetermined number of seconds will be added to that time to take intoaccount of heat dissipation during the power off interval. As shown inthe FIG., and by way of example, a power shutdown due to the occurrenceof an arc may be two seconds as at 312 and four seconds may be added tothe maintain temperature timer as at 314. In any event, therapy willcontinue.

Alternatively, the maximum number of arcs allowed Arc_(mxax) 308 couldbe a predetermined threshold average rate of arc occurrence.Microprocessor 20 would then determine the average rate of arcoccurrence and compare it with Arc_(mxax) at 306. When the average rateexceeded the threshold average rate, say for example, one arc per 15seconds or one arc per twenty seconds, then the therapy would end as at310.

Referring now to FIG. 14, the Impedance Control Loop 126 will bedescribed. This loop along with Temperature Control Loop 132 providesthe primary control, that is RF power modulation, of the ablationprocedure, unlike the Arc Detection Loop 120, which functions primarilyto simply turn the power off when certain conditions, such as an arcdetection, occurs. Thus, as the power is turned on, the applied powerwill be compared as at 319 against the default maximum power level forthe procedure, which as previously noted may be set by the surgeon or bythe disposable instrument 26. When the applied power is compared to thedefault maximum power at 319 and found to be below the maximum for theprocedure, the period of time that has elapsed since the last powerincrease or decrease will be determined as at 320. It is desirable towait a predetermined period of time between power level increases ordecreases to allow the tissue impedance to be evaluated at the presentRF power setting. This time constant is chosen based upon the virtualelectrode response time, that is, the length of time between the changein a parameter, here power, and the time at which a change is seen inthe virtual electrode. Where the predetermined time limit has notelapsed since the last power level increase, then the loop will enter await state at 321 until such time limit elapses.

If the applied power is at the default maximum, then the impedanceslope, that is, the rate of change of the impedance, will be comparedwith the impedance slope threshold as at 322. In addition, if thepredetermined period of time between allowed changes in Max RF Power haselapsed has elapsed, here, three seconds for purposes of illustration,then the impedance slope comparison of 322 will be conducted. If theimpedance slope is greater than the slope threshold, the maximum RFpower, which is the maximum power allowed based upon the measuredimpedance, will be reduced a predetermined amount, say twenty percentfor example, which is a predetermined percentage of the default maximumpower at 324.

If the measured impedance slope is less than the slope threshold, thenthe impedance will be compared with the impedance threshold as at 326.If the measured impedance is greater than the impedance threshold, thenthe maximum RF power will be reduced as at 324. If the measuredimpedance is less than the impedance threshold, then the maximum RFpower will be incremented a predetermined percentage of the defaultmaximum power as at 328. Control over the ablation process will thenpass to the temperature control loop 132. The level of increments anddecrements in the applied power can vary; for purposes of illustration atwenty percent increment/decrement percentage of the default maximum RFpower has been selected.

Table 7 below indicates the parameters and values that may be used withthe Loop 126.

TABLE 7 TYPICAL PARAMETER VALUE LOCATION DESCRIPTION Default Max Power100 watts Surgical Maximum power which generator Instrument can use inan ablation Power Reduction 3 sec Surgical Time between changes in RFDuration Instrument output due to impedance/soft arc detect ImpedanceThreshold 200 ohms Surgical Upper impedance threshold (Soft Arc)Instrument values > will result in power reductions Soft Arc Impedance 5ohms/sec Surgical Slope Instrument Power Reduction 20% Surgical Upondetection of soft arc, power Percent Instrument is reduced by this muchat each step. Percentage of Default Max Power.

Referring now to FIG. 15, the Temperature Control Loop 132 will bedescribed. Loop 132 will compare the temperatures measured by thesecondary temperature sensors with the thresholds established for thosesecondary sensors at 342. As noted previously, these secondary sensorswill normally be located at a distance away from current source. Forexample, where a needle electrode is being used to deliver current tothe virtual electrode formed within a liver tumor having an approximateradius of 1.5 centimeters is being ablated, it may be desirable to havesensors located at distances of 2.0, 2.5 and 3.0 centimeters from theneedle electrode. These sensors would ostensibly be located in healthytissue. While some margin of healthy tissue destruction would beacceptable, that is, an ablation zone of 2 or 2.5 centimeters in radius,minimizing the amount of healthy tissue being ablated is also desirable.Monitoring of the temperatures of such tissue is preferred so as to beable to discontinue the ablation procedure when the maximum desiredtemperature for that tissue is reached. This will help insure thatamount of healthy tissue that is ablated is kept to a minimum for aparticular procedure. Thus, if a secondary temperature threshold isexceeded, the power will be discontinued as at 344. An additionalfunction of measuring the temperatures with the secondary thermocouplesis to insure that a lesion of a desired size is created. In addition, itmay be desirable to place a secondary sensor at a position within thedesired lesion to enable the ablation of the tissue within the zonewhere a lesion is desired to be created so as to monitor the progress ofthe ablation.

If the secondary temperature thresholds are not exceeded, thetemperature measured by the primary sensor will be compared with theprimary temperature threshold at 346. If the threshold is not exceeded,then the power level will be set to the maximum power level at 348.Where the primary threshold is exceeded or is within a certainpredefined percentage of the threshold, the percentage will becalculated as at 350. Where the primary temperature exceeds thethreshold by a predetermined amount, say 10%, then the power will beshut off as at 344. It should be noted that the primary and secondarythermocouples may be checked in any order. During operation of VETAD 10such temperature checks will occur substantially simultaneously, but inany event more rapidly than the tissue will respond to RF power levelchanges.

When the primary temperature measurement does not exceed the predefinedpercentage then the time at the desired temperature will be determinedand compared against the predetermined time that the temperature is tobe maintained, which as noted may range from zero to about five minutes,with a typical nominal range of about thirty to about sixty seconds.That is, to ensure that the desired lesion is created and that thetissue is in fact dead, it is preferred that the temperature of thetissue at the primary thermocouple be maintained for a predefined timeperiod. If the predefined time period has expired, then the power willbe terminated as at 344. If the predetermined time period has notelapsed, then the power level necessary to maintain the temperature atthe threshold will be calculated at 354 either using a proportionalintegral derivative method discussed below or the alternative processdiscussed below. The maximum power level will then be compared with thiscalculated power level, termed RF Modulate, at 356. If the maximum powerlevel exceeds RF Modulate, the power will be set to equal RF Modulate at358. That is, at 354-358 the power level needed to attain and maintainthe primary threshold temperature is calculated and if that power levelhas not yet been reached, the generator will be instructed bymicroprocessor 20 to increase the power supplied to the surgicalinstrument 26 to that power level, or RF Modulate. If the maximum RFpower level is less than RF Modulate, then the power level will be setto the maximum RF power as at 360. The maximum RF power is the maximumpower level as calculated by the impedance control loop. This is themethod by which the impedance control loop overrides the temperaturecontrol loop.

The use of a proportional integral derivative to calculate a value is awell known technique. Thus, during operation the VETAD 10 may perform aprimary temperature control of RF power to reach and maintain a desiredtemperature at the primary thermocouple. This control is accomplishedthrough the following described process. This process will utilize avariety of inputs, including the desired temperature at the primarythermocouple, the current or measured temperature at the primarythermocouple, elapsed time, the maximum impedance, the proportional gainK_(p), the integral gain K_(i), and the derivative gain K_(d). Withthese inputs, the power level can be calculated as follows:

${Error\_ Integral} = {\sum\limits_{o}^{n}\;{{Error}\;(n)}}$

-   -   where the following constraints are imposed:    -   1. predetermined minimum <Error_Integral<predetermined maximum    -   2. Freeze Error Integral if applied RF power is at Maximum RF        power        Error_derivative=Error(n)−Error(n−1)

The output of this algorithm is ablation RF power and is computed asfollows:Ablation_(—) RF_Power=(max_(—) RF_Power)*(Error_proportion(n)*K_(p)+Error_Integral/K _(i)+Error_derivative*K _(d))

This calculated RF power is constrained by the maximum impedance RFpower as follows:Ablation RF Power=the lower of the (calculated ablation RF power) and(maximum impedance RF power).

The generator will change the output RF power to the new ablation RFpower. This calculation can be done as frequently as desired, such asonce per second.

Alternatively, the RF power level required to hold the primarytemperature at a desired temperature level can be determined as follows.First, the desired primary hold temperature T_(d), that is, thetemperature that is desired to be maintained to ensure first thecreation of a lesion and second the creation of a lesion of a particularsize, is specified either as a default or an input parameter.Temperatures currently measured T_(c) by the primary thermocouple arethen compared with the desired primary temperature T_(d). As T_(c),reaches the T_(d) and then equals it, the RF power will be decreased apredetermined amount such as 50 percent. With each predeterminedtemperature increase, such as 0.5° C., then, the applied RF power can bereduced in additional predetermined increments, such as 50 percent. Thisroutine can be followed until the applied power is reduced to one watt.As T_(c) is reduced to below T_(d), the applied RF power can beincreased by a predetermined amount, such as 50 percent. Again, as thetemperature continues to drop by a predetermined increment, the appliedpower can be increased by a predetermined amount. These temperature andpower increase increments can also be 0.5° C. and 50 percent,respectively. The increments for both increasing and decreasing the RFpower will depend on the distance of the location of the primarythermocouple from the current supplying electrode, which may be anyspecified distance. A distance of 0.5 centimeters has been found to beuseful for example.

Table 8 illustrates the various parameters that can be used with thetemperature control loop 132 and typical values that can be associatedtherewith.

TABLE 8 TYPICAL PARAMETER VALUE LOCATION DESCRIPTION Primary Temp 60 degC Surgical This is the control temp. which Instrument the algorithm willtry to maintain Secondary Temp 1 45 deg. C Surgical This is temp for oneof safety Instrument thermocouples - temp > will cause ablation to endSecondary Temp 2 45 deg. C Surgical Safety Thermocouple InstrumentSecondary Temp 3 45 deg. C Surgical Safety Thermocouple InstrumentSecondary Temp 4 45 deg. C Surgical Safety Thermocouple InstrumentSecondary Temp 5 45 deg. C Surgical Safety Thermocouple InstrumentMaintain Time 1 minute Surgical Time to maintain primary temp Instrumentonce it is achieved Overshoot % 10% Surgical Overshoot allowed onprimary Instrument/Table temp. > than this and ablation ends MaximumAblation Time 10 min or Surgical Maximum ablation time. less Instrumentand Upper limit of 10 minutes per Table/Hardware generator, but smallervalue may be programmed in Surgical Instrument Max RF Power variablesoftware variable Maximum power that can be used by the TemperatureControl Loop. Variable is controlled by the Impedance Control of RFPower Loop. Voltage Clamp 100 volts Surgical This is the maximum voltageInstrument that can be used in an ablation. Supersedes Max RF power(reaching Voltage Clamp may prevent Max Power from being delivered)Auxiliary Thermocouple No Surgical Informs generator if an PresentInstrument Auxiliary Thermocouple will be used with the disposable.(Yes, No, or Optional)

Operational control of VETAD 10 can be provided by impedance monitoring,temperature monitoring, or both. One example of an impedance monitoringloop is shown in FIG. 14, as has been discussed previously. A moredetailed example of an impedance monitoring control loop is illustratedin FIG. 16. Preferably, impedance monitoring will include both rapid,transient spikes in the measured impedance within a predetermined timeperiod as well as slower, yet continuous increases in the measuredimpedance within a longer period of time. Thus, VETAD 10 will monitornot only the absolute impedance measured at any one time, but also therate of change in the impedance over at least one and preferably twotime intervals of differing lengths to account for transient spikes inthe impedance, due to factors such as air bubbles in the conductivefluid, as well as slower rises in the impedance, which may be indicativeof the application of too much radio frequency power or a low fluidinfusion rate.

Thus, as seen in FIG. 16, VETAD 10 will take frequent impedancemeasurements. The impedance control loop 400 will include determiningthe time since the RF power level was last changed as at 402. Apredefined or default minimum time period between changes to RF powerlevels will be established as at 404. The time period 402 can then becompared with the predefined time period as at 406. If the time 402 isless than the predetermined minimum 404, then the apparatus 10 willenter a wait state as at 408. If the time 402 is greater than theminimum time T then the control loop 400 will enter the sub routine 410.

In subroutine 410 the impedance measurement circuit 90 (FIG. 3) willmeasure a first impedance Z₁ at a time t₁ at 412 and a second impedanceZ₂ at a time t₂ as at 414. Z₁ and Z₂ will be compared at 416 to providean impedance difference ΔZ₁₂. Alternatively, several ΔZ₁₂ values couldbe averaged to arrive at a calculated ΔZ₁₂. A Maximum ΔZ₁₂ over apredetermined time period, say 0.2 seconds or 200 milliseconds by way ofexample only, will be provided as a default as at 418. Alternatively,several ΔZ₁₂ values could be averaged. The calculated ΔZ₁₂ will becompared with the Maximum ΔZ₁₂ as at 420. If the calculated ΔZ₁₂ doesnot exceed the Maximum ΔZ₁₂, then the RF power level may be increased bya predetermined amount, say twenty percent by way of example, of theMaximum RF Level as at 422, the Maximum RF Level being supplied by theoperator or as an input provided by memory 42 of surgical instrument 26as at 424. This newly calculated RF power level will be supplied as at426 as an input to the Temperature Control Loop 132.

Where the calculated ΔZ₁₂ exceeds the Maximum ΔZ₁₂, then RF power may becontrolled and hence reduced as at 428 by a predetermined amount, whichby way of example may be twenty percent of the Maximum RF level 424.This calculated reduced power level will be supplied as an input to theTemperature Control Loop 132 as at 426.

In addition, a maximum impedance Z_(max), will be specified by theoperator or the memory 42 as at 430. The measured impedance Z₂ at timet₂ at 432 will be compared against Z_(max) as at 434. Where Z₂ is lessthan Z_(max) the RF power level will be increased as at 422. Where Z₂ isgreater than Z_(max) the RF power level will be decreased as at 428.

A temperature control flow chart in accord with the present invention isillustrated in FIG. 17. As seen there, a plurality of thermocouples,including a primary and secondary thermocouples, are used to measure thetissue being ablated and the temperature of the tissues surrounding thetissue to be ablated. This method of creating a virtual electrode andcontrolling the application of radio frequency power to the patient willbe operable in an automatic mode of operation of VETAD 10. Where VETAD10 is operated in a manual mode of operation, the temperature controlalgorithm of the VETAD 10 will not operate except for the function ofshutting off the application of radio frequency power to the patient ifa maximum temperature threshold is exceeded.

Referring to FIG. 17, then, VETAD 10 will include a primary temperaturecontrol 450 as determined by the temperatures measured by the primarythermocouple 452 and the secondary thermocouples 454. In operation,VETAD 10 will increase the radio frequency power applied to the tissueto be ablated until either the temperature measured by the primarythermocouple reaches the predetermined temperature range required forthe ablation of the tissue or until the predetermined maximum powerlevel for the ablation procedure to be performed is reached, it beingunderstood that different tissues may require differing levels of radiofrequency power to accomplish the desired ablation. The temperaturemeasured by primary thermocouple 452, which may be placed directlywithin the tissue to be ablated, will be compared against thepredetermined maximum temperature 456 as indicated at 458.

If the measured temperature of the primary thermocouple is less than themaximum temperature for that thermocouple for that particular procedure,then the power will continue to be increased to the Maximum RF PowerLevel 460 as provided for the control loop 400 as indicated at 460.Where the temperature measured by the primary thermocouple equals orexceeds the maximum temperature for the procedure, then the achievedtemperature will be maintained by controlling the applied power as at462. Thus, if the achieved temperature exceeds the maximum temperature,the applied power will be modulated and the power will generally bereduced as necessary to bring the measured temperature at or just belowthe desired temperature. If the measured temperature is less than theminimum desired temperature, however, then the applied power will beincreased. The power level will be set as at 464 for application topatient 28.

Once the preferred minimum temperature for the procedure is reached, theduration of the maintenance of that temperature will be compared as at466 with default or user defined time period 468 for the temperatureduration. That is, to ensure the creation of the desired lesion size andthe consequent death of the desired tissue, the desired temperatureshould be maintained for a predetermined period of time based upon thetime of tissue being ablated. Once the time period for the temperaturemaintenance has been achieved, then the radio frequency power will beturned off as indicated at 464. Alternatively, the temperature of thetarget tissue to be ablated can be increased to a predeterminedtemperature above the temperature for cell death and then the RF powercan be discontinued. The latent heat in the tissue will be dissipatedprimarily by cell-to-cell conduction, thus ensuring that the cell deathtemperature will be exceeded by the target tissue for some period oftime. Creation of the desired lesion size will then depend upon thetemperature reached by the tissue.

If secondary thermocouples 454 are used to monitor the temperature ofpreselected tissue during the ablation procedure, then if any one of thetemperature measurements exceeds its prescribed maximum as at 470 asinput manually by the operator, held in memory 472 in VETAD 10, or heldin memory 42 of surgical instrument 26, then the applied power will bediscontinued as at 464. In this way, the size of the lesion can becontrolled such that the minimum of healthy tissue is affected by theablation procedure. Alternatively, certain procedures may allow theoperator to reduce the applied RF power rather than discontinue it.

During a therapy procedure the physician may find it desirable tomanually discontinue the application of RF power for preselected periodof times, particularly when operating VETAD 10 in the manual mode. Thusa therapy can enter a “pause” mode that may be activated, for example,by release of the foot pedal 62 during RF treatment. The therapy canenter a “resume” mode wherein the application of RF power isreinitiated, for example, by re-pressing the foot pedal 62 within apredetermined period of time, for example, 15 seconds, after a “pause”was initiated.

Discontinuing the application of RF power will enable the patient's bodyto immediately begin to dissipate the heat built up in the tissue, thuscooling the tissue whose ablation is desired. This cooling effect willbe particularly true where, as in most cases, the conductive fluid isallowed to continue to flow, thereby carrying heat away from theablation site. If therapy is to be resumed, the dissipation of the heatduring the pause should be accounted for to ensure that the therapy issuccessful. As has been discussed generally earlier, a maximum timeperiod will generally be established for a particular procedure toensure the safety of the patient. Thus, the time spent in the pause modemust be accounted for to ensure that an ablation procedure can becompeted and to account for the heat dissipation. Several methods may beused to account for this time spent in the pause mode, including:t _(resume)=1.5t _(pause) +t _(continue)t _(resume) =t _(reheat) +t _(continue)where:

-   -   t_(resume)=total RF time after therapy cycle, e.g., that is, the        foot pedal, is reactivated until complete cycle is completed.    -   t_(pause)=time during “pause” mode    -   t_(reheat)=time to reach T_(last) after therapy enters “resume”        mode following a pause    -   t_(continue)=remaining time of RF to complete initial setting        t_(total)    -   t_(total)=RF time setting for the therapy    -   t_(pre-pause)=RF time until “pause”    -   T_(last)=last temperature measured at selected tissue        thermocouple before “pause”    -   T=temperature measured at selected tissue thermocouple    -   T_(body)=body temperature

The following conditions would be assumed:If during t_(pause), T=T_(body), then restart at t_(total);andIf t_(reheat)+t_(continue) reaches t_(total), then RF power is shut off.

The first method of determining the time remaining in the therapysession for the application of RF power, t_(resume), is to add the timespent during the pause mode, t_(pause), plus a percentage thereof to theamount of time remaining to complete the original prescribed therapy,t_(continue).

The second method shown above for determining the time remaining in thepause mode spent in the pause mode is to measure the time t_(reheat)spent in reaching the last temperature T_(last) recorded before RF powerwas discontinued once therapy has resumed and adding that time to theamount of time remaining to complete the original prescribed therapy,t_(continue).

In addition, t_(resume) could be determined as follows:t _(resume) =t _(pause)+(database library of time)where the “database library of time” comprises a table of experimentaldata of time to be added to the remaining “resume” time to provide aneffective treatment based upon t_(pause) time in relation to treatmentcycle.

The first condition states that if the pause is so long that the tissuebeing ablated once again cools to body temperature, then the therapycycle should begin anew with a complete time cycle. The second conditionstates that if the time to reheat the tissue to T_(last) and the timeremaining in the therapy, t_(continue) is equal to the total time forthe therapy then the therapy should be discontinued.

Either of the foregoing methods of determining the amount of timeremaining t_(resume) in the therapy cycle will aid in providing a safeand efficacious therapy for the patient where the physician decides forwhatever to initiate an indeterminate halt in the application of RFpower during a therapy session.

Referring now to FIGS. 18-20, the operation and method of the presentinvention will be explained more fully. FIGS. 18-20 are highly schematicillustrations and are meant to be exemplary the operation and method ofthe present invention. As seen in FIG. 18, a surgical instrumentcomprising a straight needle 500 has been inserted percutaneously, thatis, through the skin 502, shown in phantom outline, into a tissue 504 tobe ablated, for example, a liver metastases, in a patient's liver 506.Needle 500 will be fluidly and electrically connected to the VETAD 10 bya line 508. Needle 500 will preferably have a thermocouple 510 disposedat the distal end thereof. After placement of the needle 500 at thedesired location, one or more additional thermocouples 512 may be placedto provide additional or secondary temperature monitoring and connectedto VETAD 10 by a line 514. One or more of these thermocouples could beplaced in the metastases 504 remote from the needle 500 or they could beplaced in tissue surrounding the metastases.

Referring now to FIG. 19, an enlarged view of a needle useful with thepresent invention will be described. It will be observed that the needle500 can be hollow, that is, include an interior lumen or passage 520shown in phantom. The conductive fluid will be pumped through lumen 520to one or more apertures 522, through which the conductive fluid mayflow into the tissue to be ablated 504. As illustrated in the FIG., theneedle 500 includes a single aperture 522 at the distal end thereof.Alternatively, the single distal aperture could be plugged and aplurality of apertures placed in the side surface 524 of the needle, orboth. It will be understood that other surgical instruments can also beused with the present invention.

Once the needle has been placed within the tissue, the pre-ablationinfusion of conductive fluid will begin. The infusion of the conductivefluid will create an interstitial virtual electrode. Referring to FIG.20, then, it will be observed that the conductive fluid has permeatedthe tissue 504 interstitially so as to create a virtual electrode 530shown in phantom. Once the desired level of pre-ablation infusion hasoccurred, or stated otherwise, once the desired virtual electrode sizehas been approximately achieved, the radio frequency current provided bythe generator 18 will be applied to the tissue 504 through the needle500, which serves as a metal electrode as well as a conductive fluiddelivery port. The virtual electrode 530 may have a substantiallyspherical or oval shape; the exact configuration of the virtualelectrode will depend upon factors such as tissue irregularities,channels between cells, and the direction in which the fluid flow isdirected and any differential fluid flow in a particular direction,among others. The virtual electrode 530 will, as previously described,enable the applied current density to be spread over a large volume oftissue and thus will create a larger lesion than can be achieved with“dry” radio frequency power. The tissue ablation procedure will becontrolled as previously described above. The conductive or electrolyticfluid will be supplied to the tissue throughout the ablation procedure,that is, at least as long as the RF current is being provided thereto.

It will be observed that the foregoing description of the presentinvention has spoken often and generally of measuring impedance.Impedance, as is well known, is the resistance to an alternating currentbetween two locations along a current pathway. Generally speaking, thepresent invention measures the impedance between two locations withinthe apparatus 10 along the RF current pathway. Nevertheless, theresistance to the current flow between the apparatus and the virtualelectrode and between the ground pad (or other return electrode where abipolar instrument is used) and the apparatus forms a small, reasonablywell known, and non-varying resistance that can be considered in theimpedance measurements that are made. The impedance that varies is thatbetween the tissue-virtual electrode interface and the tissue-ground padinterface, with the bulk of the impedance being at the tissue-virtualelectrode interface. Thus, it will be understood that, in essence, it isthe impedance at the tissue-virtual electrode interface that ismonitored and of concern. By “considered” it is meant that the varyingparameters relating to the impedance measurements can either beincreased to account for the impedance along the current pathway atother than the tissue-virtual electrode interface, decreased, or setwith that value in mind.

While the use of a straight needle as a surgical instrument has beenshown and described, it will be understood that-the present invention isnot so limited. For example, a hollow helical needle as shown in U.S.Pat. No. 5,431,649 to Mulier, et al. could also be used with the presentinvention. In addition, the present invention could also be used withsurface devices such as forceps or a rollerball, as known in the art. Asnoted, the interconnection of the disposable surgical instrument 26 withthe VETAD 10 can be “coded” with the number of active pins such that theVETAD 10 can recognize and identify the particular surgical instrumentbeing used according to its programming and establish predetermineddefault parameters for use of a particular instrument 26 or theinstrument 26 can include a memory chip holding default operatingparameters that are provided to the microprocessor 20 when theinstrument 26 is queried by the microprocessor. The present invention isthus not limited to a single type of surgical instrument. A surgicalinstrument useful with the present invention, however, will be able todeliver a conductive fluid to a desired ablation site as well as a radiofrequency current.

The present invention has been noted as being able to use saline as aconductive fluid. The present invention is not so limited, however, andother conductive solutions that are not toxic in the amounts to be usedduring an ablation procedure may also be used therewith. In addition,contrast fluids such as Hypaque™ may be used in connection with otherconductive fluids to provide the ability to image the creation, control,and operation of the virtual electrode during a procedure.

As previously noted the present invention finds use with conductive orelectrolytic solutions. As an example, saline in either its isotonic orhypertonic form has been found to be useful in the RF ablation process.Desirably, the solution used during an RF procedure should include aconductive medium and a buffer to provide a proper PH. In addition, asolution used in accord with the present invention could include acontrast media to allow fluoroscopic imaging of the virtual electrodeand a preservative to extend the shelf life of any prepared solution.Finally where a malignancy is being ablated, a cytotoxin could be addedto the solution to enhance the tissue killing effects of the ablationprocess.

Reference can be made to Table 9 below, which shows various componentsof an RF ablation solution that may satisfy the foregoing criteria.Preferably, an RF ablation solution will have an initial impedance ofgreater than or about 200 ohms at room temperature. As noted previouslyin the discussion of how an RF ablation process is carried out, as RFenergy is applied to the tissue through the electrolytic solution, thetissue begins to heat. Desirably, as the temperature of the solutionrises, the impedance of the solution will decrease. A decrease to animpedance level of less than 100 ohms at the termination of theprocedure is desirable. The decrease in impedance allows greatercoupling of RF ablating energy to the tissue as the temperature rises.This in turn enables the ablation process to be carried out more rapidlythan if the impedance of the conductive fluid remained constant ornearly so as the temperature increased. Reducing the time for theablation process to be carried out is desirable from a clinic standpointto minimize the effects of the process on the patient.

Referring to the tables below, it should be understood that theformulations appearing in Table 9 are representative of the varioustypes of chemicals that can be used in an RF ablating solution and arenot meant to indicate that the particular chemicals shown therein mustbe used in a horizontal fashion across the table. Stated otherwise forexample, an RF ablating solution may include an electrolyte comprisingone of sodium chloride, sodium bicarbonate, or magnesium chloride; oneof a cytotoxin comprising barium chloride, cisplatin or alcohol; acontrast agent comprising one of any generally acceptable iodineproduct, hypaque, omnipaque, or conray; a buffer, if necessary, such ashydrogen chloride; and if necessary, a stabilizer such as propyleneglycol. It will be understood that where the RF ablating solution is notbeing used to ablate malignancies that the cytotoxin would in mostinstances be neither necessary nor desirable. Such uses would includeablation of prostate tissue or other tissue where no malignancy wasindicated and surface rather than interstitial ablation of tissues insuch procedures as, lung volume reduction, anuloplasty, treatment ofvascular abnormalities such as but not limited to aneurysm,arteriovenous malformations, fistulas, sterilization procedures throughclosure of the fallopian tubes, and treatment of varicose veins.

Referring now to Table 10, specific formulations of RF ablatingsolutions are shown for particular tissue being treated. It will beunderstood that the relative concentrations of each of the chemicalcomponents forming the solution can be varied and fall generally withina broadly acceptable range. For example, the concentration of theelectrolyte itself can vary from isotonic saline at 0.9% to hypertonicsaline at up to 37%. For example, where the tumor being treated isgreater than 3.5 centimeters in diameter, hypertonic saline is useful inthe treatment. Where the tumor size is generally 1 centimeter or less,isotonic saline can be used as the electrolyte. For tumors fallingwithin the range of about 1 centimeter to about 3.5 centimeters theconcentration of the saline can vary between isotonic and hypertonicconcentrations. As previously noted, generally it is desirable to have areduction in impedance of the RF ablating solution of at least 50% fromthe start of the procedure to its termination. Thus, the concentrationsof the various components of the RF ablating solution are desirablyadjusted so as to provide the desired impedance drop as well as performthe necessary functions for which they are provided. Thus, for example,where alcohol is being used as the cytotoxic agent the concentration ofthe electrolyte will desirably be increased since alcohol itself is apoor conductor and will adversely affect the impedance performance ofthe solution otherwise.

TABLE 9 Contrast Electrolyte Cytotoxin Agent Buffer Stabilizer NaCl BaClIodine HCl Propylene glycol generally Na(CO₃)₂ Cisplatin Hypaque MgClAlcohol Omnipaque Conray

TABLE 10 Stabilizer/ Contrast Pre- Tissue Electrolyte Cytotoxin MediumBuffer servative Tumor Hypertonic BaCl Iodine HCl CaNa₂ 3.5 cm NaCl 1 cmTumor HCl 3.5 cm Tumor 1 cm Isoasmotic Cisplatin HCl Propylene NaClglycol Strabismus Isoasmotic Not Not HCl Propylene NaCl applicableapplicable glycol Liver NaCl Not Not HCl CaNa₂ applicable applicableLung Isoasmotic Not Not HCl CaNa₂ NaCl applicable applicable

The present invention has been described relative to an apparatusillustrated with a single fluid supply. As previously noted, however,the present invention could include a plurality of such fluidadministration systems. For example, a separate fluid supply/pump systemfor the administration of an insulating fluid such as dextrose could beprovided. This fluid would be provided to tissue whose ablation was notdesired. The dextrose would substantially prevent current transfer tothat tissue thereby preventing its ablation. Alternatively, a chilledfluid could be provided to tissue whose ablation was not desired. Insuch a circumstance the chilled fluid would act as a heat sink to carryheat away from such tissue. The administration of the chilled orinsulated fluid would also be controlled by the microprocessor 20according to predetermined parameters. Also, several fluid sources maybe used to apply independently varying flow rates to different locationson a more complex electrode configuration. In addition, where fluid isbeing supplied to tissue through a needle having one or more smallapertures, such apertures can become clogged or plugged, leading to thetermination of the procedure due to a high impedance conditiondeveloping. Supplying fluid to multiple needles from multiple fluidsources will lessen the likelihood that such a high impedance conditionwill develop and that the procedure will have to be terminated. Thus, insummary, the present invention contemplates a plurality of fluid pumps,such as syringe pumps or flexible tube pumps, controlled bymicroprocessor 20, each pump connected to a separate fluid supply so asto provide the physician with the ability to administer a variety offluids during an ablation procedure.

It will further be understood that while the present invention has beendescribed as being used only relative to a liver metastases, that it isnot so limited. Thus, the present invention may be useful for ablatingcardiac tissue responsible for irregular heartbeats and for treatment ofbenign prostate hypertrophy. In addition to liver metastases, otherneoplasms, such as breast, lung, and prostate cancer may also be treatedin accord with the present invention. It may also be used in thetreatment of vascular disease including arteriovenous malformations andaneurysms. In addition, the present invention may be used to treat softtissue, such as the lung, where a reduction in lung volume is desired,and solid organs and tissues, among them bone.

Additionally, it will be understood that the present invention is usefulwith either a monopolar or bi-polar electrode. Thus, where a monopolarelectrode is used, a ground electrode will be placed in contact with thepatient as is well known in the prior art to provide a complete circuitfor the applied radio frequency current. That is, the ground electrodewill be connected to the apparatus 10 and the current will travel fromthe generator 18 to the surgical instrument 26, then through the patientto the ground electrode and back to the apparatus 10. Where a bipolarelectrode is used the current path will be from the apparatus to thesurgical instrument to the patient and then back to the surgicalinstrument and the apparatus.

Finally, it will be understood that the present invention can be used totreat both human and non-human patients.

The present invention having thus been described, other modifications,alterations, or substitutions may now suggest themselves to thoseskilled in the art, all of which are within the spirit and scope of thepresent invention. It is therefore intended that the present inventionbe limited only by the scope of the attached claims below.

1. A method for treating tissue comprising: providing radio frequencypower at a power level; providing a fluid at a fluid flow rate;controlling the fluid flow rate based upon the power level; and applyingthe radio frequency power with the fluid to treat the tissue from asurgical instrument.
 2. The method of claim 1, wherein: the fluid flowrate is controlled by use of an equation.
 3. The method of claim 2,wherein: the equation comprises a quadratic equation.
 4. The method ofclaim 2, wherein: the equation comprises a higher order equation.
 5. Themethod of claim 1, wherein: the fluid flow rate is controlled by use ofa linear relationship.
 6. The method of claim 1, wherein: the fluid flowrate is controlled by use of a step relationship.
 7. The method of claim1, wherein: the fluid flow rate is controlled by use of a look-up table.8. The method of claim 1, wherein: controlling the fluid flow rate basedupon the power level is performed by a control apparatus comprising amicroprocessor.
 9. The method of claim 1, wherein: the surgicalinstrument comprises a monopolar surgical instrument.
 10. The method ofclaim 1, wherein: the surgical instrument comprises a bipolar surgicalinstrument.
 11. method of claim 1, wherein: the radio frequency power isprovided from a radio frequency power source, and the radio frequencypower source is configured to provide the radio frequency power in apower range between and including about 0.1 watts to about 200 watts.12. The method of claim 11, wherein: the radio frequency power sourcecomprises a radio frequency generator.
 13. The method of claim 11,wherein: the radio frequency power source is configured to provide theradio frequency power at the power level in a frequency range betweenand including about 350 kHz to about 700 kHz and a resistive load rangebetween and including about 10 ohms to about 500 ohms.
 14. The method ofclaim 1, wherein: the power level is provided in a range between andincluding about 0.1 watts to about 200 watts.
 15. The method of claim 1,wherein: the fluid flow rate is provided in a range between andincluding about 0.1 cubic centimeters per minute to about 10 cubiccentimeters per minute.
 16. The method of claim 1, wherein: the fluidcomprises at least one of an electrically conductive fluid and anelectrolytic fluid.
 17. The method of claim 1, wherein: the fluidcomprises at least one of an electrically conductive solution and anelectrolytic solution.
 18. The method of claim 1, wherein: the fluidcomprises saline.
 19. The method of claim 1, wherein: the fluid isprovided from a fluid source comprising a container.
 20. The method ofclaim 19, wherein: the container comprises a flexible bag or a syringe.21. The method of claim 1, wherein: the radio frequency power isprovided from a radio frequency power source; the fluid is provided froma fluid source; controlling the fluid flow rate based upon the powerlevel is performed by a control apparatus comprising a microprocessor;and the surgical instrument comprises an electrosurgical instrument inelectrical communication with the radio frequency power source and influid communication with the fluid source, the electrosurgicalinstrument to provide the radio frequency power from the power sourceand the fluid from the fluid source to treat the tissue.
 22. The methodof claim 21, wherein: the fluid flow rate used at the power level isstored in a memory.
 23. The method of claim 21, wherein: theelectrosurgical instrument is in fluid communication with the fluidsource via a fluid line.
 24. The method of claim 23, wherein: a pump isused to convey the fluid through the fluid line.
 25. The method of claim24, wherein: the pump comprises a syringe pump.
 26. The method of claim24, wherein: the pump comprises a flexible tube pump.
 27. The method ofclaim 24, wherein: the pump is configured to force the fluid out of thefluid source.
 28. The method of claim 1, wherein: the tissue comprisessoft tissue.
 29. The method of claim 1, wherein: the tissue comprisesbone tissue.
 30. The method of claim 1, wherein: the tissue comprises atleast one of cardiac tissue, liver tissue, lung tissue, prostate tissue,breast tissue, organ tissue, vascular tissue and neoplasm tissue.
 31. Amethod for treating tissue comprising: providing radio frequency powerat a power level; providing a fluid at a fluid flow rate; providing arelationship between the fluid flow rate and the power level; andapplying the radio frequency power with the fluid to treat the tissuefrom a surgical instrument.
 32. The method of claim 31, wherein: therelationship is provided by an equation.
 33. The method of claim 32,wherein: the equation comprises a quadratic equation.
 34. The method ofclaim 32, wherein: the equation comprises a higher order equation. 35.The method of claim 31, wherein: the relationship comprises a linearrelationship.
 36. The method of claim 31, wherein: the relationshipcomprises a step relationship.
 37. The method of claim 31, wherein: therelationship is provided by a look-up table.
 38. The method of claim 31,further comprising: the relationship between the fluid flow rate and thepower level is used by a control apparatus comprising a microprocessor.39. The method of claim 31, wherein: the surgical instrument comprises amonopolar surgical instrument.
 40. The method of claim 31, wherein: thesurgical instrument comprises a bipolar surgical instrument.
 41. Themethod of claim 31, wherein: the surgical instrument comprises a bipolarsurgical instrument.
 42. The method of claim 31, wherein: the radiofrequency power is provided from a radio frequency power source, and theradio frequency power source is configured to provide the radiofrequency power in a power range between and including about 0.1 wattsto about 200 watts.
 43. The method of claim 42, wherein: the radiofrequency power source comprises a radio frequency generator.
 44. Themethod of claim 42, wherein: the radio frequency power source isconfigured to provide the radio frequency power at the power level in afrequency range between and including about 350 kHz to about 700 kHz anda resistive load range between and including about 10 ohms to about 500ohms.
 45. The method of claim 31, wherein: the power level is providedin a range between and including about 0.1 watts to about 200 watts. 46.The method of claim 31, wherein: the fluid flow rate is provided in arange between and including about 0.1 cubic centimeters per minute toabout 10 cubic centimeters per minute.
 47. The method of claim 31,wherein: the fluid comprises at least one of an electrically conductivefluid and an electrolytic fluid.
 48. The method of claim 31, wherein:the fluid comprises at least one of an electrically conductive solutionand an electrolytic solution.
 49. The method of claim 31, wherein: thefluid comprises saline.
 50. The method of claim 31, wherein: the fluidis provided from a fluid source comprising a container.
 51. The methodof claim 50, wherein: the container comprises a flexible bag or asyringe.
 52. The method of claim 31, wherein: the radio frequency poweris provided from a radio frequency power source; the fluid is providedfrom a fluid source; the relationship between the fluid flow rate andthe power level is used by a control apparatus comprising amicroprocessor; and the surgical instrument comprises an electrosurgicalinstrument in electrical communication with the radio frequency powersource and in fluid communication with the fluid source, theelectrosurgical instrument to provide the radio frequency power from thepower source and the fluid from the fluid source to treat the tissue.53. The method of claim 52, wherein: the fluid flow rate used at thepower level is stored in a memory.
 54. The method of claim 52, wherein:the electrosurgical instrument is in fluid communication with the fluidsource via a fluid line.
 55. The method of claim 54, wherein: a pump isused to convey the fluid through the fluid line.
 56. The method of claim55, wherein: the pump comprises a syringe pump.
 57. The method of claim55, wherein: the pump comprises a flexible tube pump.
 58. The method ofclaim 55 wherein: the pump is configured to force the fluid out of thefluid source.
 59. The method of claim 31, wherein: the tissue comprisessoft tissue.
 60. The method of claim 31, wherein: the tissue comprisesbone tissue.
 61. The method of claim 31, wherein: the tissue comprisesat least one of cardiac tissue, liver tissue, lung tissue, prostatetissue, breast tissue, organ tissue, vascular tissue and neoplasmtissue.
 62. A method for treating tissue comprising: providing radiofrequency power at a power level; providing a fluid at a fluid flowrate; relating the fluid flow rate to the power level; and applying theradio frequency power with the fluid to treat the tissue from a surgicalinstrument.
 63. The method of claim 62, wherein: the fluid flow rate isrelated by use of an equation.
 64. The method of claim 63, wherein: theequation comprises a quadratic equation.
 65. The method of claim 63,wherein: the equation comprises a higher order equation.
 66. The methodof claim 62, wherein: the fluid flow rate is related by use of a linearrelationship.
 67. The method of claim 62, wherein: the fluid flow rateis related by use of a step relationship.
 68. The method of claim 62,wherein: the fluid flow rate is related by use of a look-up table. 69.The method of claim 62, wherein: relating the fluid flow rate to thepower level is performed by a control apparatus comprising amicroprocessor.
 70. The method of claim 62, wherein: the surgicalinstrument comprises a monopolar surgical instrument.
 71. The method ofclaim 62, wherein: the radio frequency power is provided from a radiofrequency power source, and the radio frequency power source isconfigured to provide the radio frequency power in a power range betweenand including about 0.1 watts to about 200 watts.
 72. The method ofclaim 71, wherein: the radio frequency power source comprises a radiofrequency generator.
 73. The method of claim 71, wherein: the radiofrequency power source is configured to provide the radio frequencypower at the power level in a frequency range between and includingabout 350 kHz to about 700 kHz and a resistive load range between andincluding about 10 ohms to about 500 ohms.
 74. The method of claim 62,wherein: the power level is provided in a range between and includingabout 0.1 watts to about 200 watts.
 75. The method of claim 62, wherein:the fluid flow rate is provided in a range between and including about0.1 cubic centimeters per minute to about 10 cubic centimeters perminute.
 76. The method of claim 62, wherein: the fluid comprises atleast one of an electrically conductive fluid and an electrolytic fluid.77. The method of claim 62, wherein: the fluid comprises at least one ofan electrically conductive solution and an electrolytic solution. 78.The method of claim 62, wherein: the fluid comprises saline.
 79. Themethod of claim 62, wherein: the fluid is provided from a fluid sourcecomprising a container.
 80. The method of claim 79, wherein: thecontainer comprises a flexible bag or a syringe.
 81. The method of claim62, wherein: the radio frequency power is provided from a radiofrequency power source; the fluid is provided from a fluid source;relating the fluid flow rate to the power level is performed by acontrol apparatus comprising a microprocessor; and the surgicalinstrument comprises an electrosurgical instrument in electricalcommunication with the radio frequency power source and in fluidcommunication with the fluid source, the electrosurgical instrument toprovide the radio frequency power from the power source and the fluidfrom the fluid source to treat the tissue.
 82. The method of claim 81,wherein: the fluid flow rate used at the power level is stored in amemory.
 83. The method of claim 81, wherein: the electrosurgicalinstrument is in fluid communication with the fluid source via a fluidline.
 84. The method of claim 83, wherein: a pump is used to convey thefluid through the fluid line.
 85. The method of claim 84, wherein: thepump comprises a syringe pump.
 86. The method of claim 84, wherein: thepump comprises a flexible tube pump.
 87. The method of claim 84,wherein: the pump is configured to force the fluid out of the fluidsource.
 88. The method of claim 62, wherein: the tissue comprises softtissue.
 89. The method of claim 62, wherein: the tissue comprises bonetissue.
 90. The method of claim 62, wherein: the tissue comprises atleast one of cardiac tissue, liver tissue, lung tissue, prostate tissue,breast tissue, organ tissue, vascular tissue and neoplasm tissue.
 91. Amethod for treating tissue comprising: providing radio frequency powerat a power level; providing a fluid at a fluid flow rate; applying theradio frequency power with the fluid to treat the tissue from a surgicalinstrument; and adjusting the fluid flow rate based upon the powerlevel.
 92. The method of claim 91, wherein: the fluid flow rate isadjusted by use of an equation.
 93. The method of claim 92, wherein: theequation comprises a quadratic equation.
 94. The method of claim 92,wherein: the equation comprises a higher order equation.
 95. The methodof claim 91, wherein: the fluid flow rate is adjusted by use of a linearrelationship.
 96. The method of claim 91, wherein: the fluid flow rateis adjusted by use of a step relationship.
 97. The method of claim 91,wherein: the fluid flow rate is adjusted by use of a look-up table. 98.The method of claim 91, wherein: adjusting the fluid flow rate basedupon the power level is performed by a control apparatus comprising amicroprocessor.
 99. The method of claim 91, wherein: the surgicalinstrument comprises a monopolar surgical instrument.
 100. The method ofclaim 91, wherein: the surgical instrument comprises a bipolar surgicalinstrument.
 101. The method of claim 91, wherein: the radio frequencypower is provided from a radio frequency power source, and the radiofrequency power source is configured to provide the radio frequencypower in a power range between and including about 0.1 watts to about200 watts.
 102. The method of claim 101, wherein: the radio frequencypower source comprises a radio frequency generator.
 103. The method ofclaim 101, wherein: the radio frequency power source is configured toprovide the radio frequency power at the power level in a frequencyrange between and including about 350 kHz to about 700 kHz and aresistive load range between and including about 10 ohms to about 500ohms.
 104. The method of claim 91, wherein: the power level is providedin a range between and including about 0.1 watts to about 200 watts.105. The method of claim 91, wherein: the fluid flow rate is provided ina range between and including about 0.1 cubic centimeters per minute toabout 10 cubic centimeters per minute.
 106. The method of claim 91,wherein: the fluid comprises at least one of an electrically conductivefluid and an electrolytic fluid.
 107. The method of claim 91, wherein:the fluid comprises at least one of an electrically conductive solutionand an electrolytic solution.
 108. The method of claim 91, wherein: thefluid comprises saline.
 109. The method of claim 91, wherein: the fluidis provided from a fluid source comprising a container.
 110. The methodof claim 109, wherein: the container comprises a flexible bag or asyringe.
 111. The method of claim 91, wherein: the radio frequency poweris provided from a radio frequency power source; the fluid is providedfrom a fluid source; adjusting the fluid flow rate based upon the powerlevel is performed by a control apparatus comprising a microprocessor;and the surgical instrument comprises an electrosurgical instrument inelectrical communication with the radio frequency power source and influid communication with the fluid source, the electrosurgicalinstrument to provide the radio frequency power from the power sourceand the fluid from the fluid source to treat the tissue.
 112. The methodof claim 111, wherein: the fluid flow rate used at the power level isstored in a memory.
 113. The method of claim 111, wherein: theelectrosurgical instrument is in fluid communication with the fluidsource via a fluid line.
 114. The method of claim 113, wherein: a pumpis used to convey the fluid through the fluid line.
 115. The method ofclaim 114, wherein: the pump comprises a syringe pump.
 116. The methodof claim 114, wherein: the pump comprises a flexible tube pump.
 117. Themethod of claim 114, wherein: the pump is configured to force the fluidout of the fluid source.
 118. The method of claim 91, wherein: thetissue comprises soft tissue.
 119. The method of claim 91, wherein: thetissue comprises bone tissue.
 120. The method of claim 91, wherein: thetissue comprises at least one of cardiac tissue, liver tissue, lungtissue, prostate tissue, breast tissue, organ tissue, vascular tissueand neoplasm tissue.
 121. A system for treating tissue comprising: aradio frequency power source to provide radio frequency power; a fluidsource to provide a fluid; an electrosurgical instrument connectable tothe radio frequency power source and the fluid source, theelectrosurgical instrument to provide the radio frequency power from thepower source and the fluid from the fluid source to treat the tissue;and a control apparatus to control a fluid flow rate of the fluid basedupon a power level of the radio frequency power.
 122. The system ofclaim 121, wherein: the control apparatus comprises a microprocessor.123. The system of claim 122, further comprising: a pump.
 124. Thesystem of claim 123, wherein: the pump, the microprocessor and the powersource are contained in a single housing.
 125. The system of claim 124,wherein: the housing further contains the fluid source.
 126. A systemfor treating tissue comprising: a radio frequency power source toprovide radio frequency power; a fluid source to provide a fluid; anelectrosurgical instrument connectable to the radio frequency powersource and the fluid source, the electrosurgical instrument to providethe fluid from the fluid source and the radio frequency power from thepower source to treat the tissue; and a control apparatus to provide arelationship between a fluid flow rate of the fluid and a power level ofthe radio frequency power.
 127. The system of claim 126, wherein: thecontrol apparatus comprises a microprocessor.
 128. The system of claim127, further comprising: a pump.
 129. The system of claim 128, wherein:the pump, the microprocessor and the power source are contained in asingle housing.
 130. The system of claim 129, wherein: the housingfurther contains the fluid source.
 131. A system for treating tissuecomprising: a radio frequency power source to provide radio frequencypower; a fluid source to provide a fluid; an electrosurgical instrumentconnectable to the radio frequency power source and the fluid source,the electrosurgical instrument to provide the fluid from the fluidsource and the radio frequency power from the power source to treat thetissue; and a control apparatus to provide a fluid flow rate of thefluid in relation to a power level of the radio frequency power. 132.The system of claim 131, wherein: the control apparatus comprises amicroprocessor.
 133. The system of claim 132, further comprising: apump.
 134. The system of claim 133, wherein: the pump, themicroprocessor and the power source are contained in a single housing.135. The system of claim 134, wherein: the housing further contains thefluid source.
 136. A system for treating tissue comprising: a radiofrequency power source to provide radio frequency power; a fluid sourceto provide a fluid; an electrosurgical instrument connectable to theradio frequency power source and the fluid source, the electrosurgicalinstrument to provide the fluid from the fluid source and the radiofrequency power from the power source to treat the tissue; and a controlapparatus to adjust a fluid flow rate of the fluid based upon a powerlevel of the radio frequency power.
 137. The system of claim 136,wherein: the control apparatus comprises a microprocessor.
 138. Thesystem of claim 137, further comprising: a pump.
 139. The system ofclaim 138, wherein: the pump, the microprocessor and the power sourceare contained in a single housing.
 140. The system of claim 139,wherein: the housing further contains the fluid source.